Molecular cloning of protein 4.1, a major structural element of the human erythrocyte membrane skeleton

The Role of Cytoskeleton Protein 4.1 in Immunotherapy

Cytoskeleton protein 4.1 is an essential class of skeletal membrane protein, initially found in red blood cells, and can be classified into four types: 4.1R (red blood cell type), 4.1N (neuronal type), 4.1G (general type), and 4.1B (brain type). As research progressed, it was discovered that cytoskeleton protein 4.1 plays a vital role in cancer as a tumor suppressor. Many studies have also demonstrated that cytoskeleton protein 4.1 acts as a diagnostic and prognostic biomarker for tumors. Moreover, with the rise of immunotherapy, the tumor microenvironment as a treatment target in cancer has attracted great interest. Increasing evidence has shown the immunoregulatory potential of cytoskeleton protein 4.1 in the tumor microenvironment and treatment. In this review, we discuss the role of cytoskeleton protein 4.1 within the tumor microenvironment in immunoregulation and cancer development, with the intention of providing a new approach and new ideas for future cancer diagnosis and treatment.

BAZ1B the Protean Protein

The bromodomain adjacent to the zinc finger domain 1B (BAZ1B) or Williams syndrome transcription factor (WSTF) are just two of the names referring the same protein that is encoded by the WBSCR9 gene and is among the 26-28 genes that are lost from one copy of 7q11.23 in Williams syndrome (WS: OMIM 194050). Patients afflicted by this contiguous gene deletion disorder present with a range of symptoms including cardiovascular complications, developmental defects as well as a characteristic cognitive and behavioral profile. Studies in patients with atypical deletions and mouse models support BAZ1B hemizygosity as a contributing factor to some of the phenotypes. Focused analysis on BAZ1B has revealed this to be a versatile nuclear protein with a central role in chromatin remodeling through two distinct complexes as well as being involved in the replication and repair of DNA, transcriptional processes involving RNA Polymerases I, II, and III as well as possessing kinase activity. Here, we provide a comprehensive review to summarize the many aspects of BAZ1B function including its recent link to cancer.

RNA-Binding Proteins PCBP1 and PCBP2 Are Critical Determinants of Murine Erythropoiesis

We previously demonstrated that the two paralogous RNA-binding proteins PCBP1 and PCBP2 are individually essential for mouse development: Pcbp1-null embryos are peri-implantation lethal, while Pcbp2-null embryos lose viability at midgestation. Midgestation Pcbp2^-/- embryos revealed a complex phenotype that included loss of certain hematopoietic determinants. Whether PCBP2 directly contributes to erythropoietic differentiation and whether PCBP1 has a role in this process remained undetermined. Here, we selectively inactivated the genes encoding these two RNA-binding proteins during differentiation of the erythroid lineage in the developing mouse embryo. Individual inactivation of either locus failed to impact viability or blood formation. However, combined inactivation of the two loci resulted in midgestational repression of erythroid/hematopoietic gene expression, loss of blood formation, and fetal demise. Orthogonal ex vivo analyses of primary erythroid progenitors selectively depleted of these two RNA-binding proteins revealed that they mediate a combination of overlapping and isoform-specific impacts on hematopoietic lineage transcriptome, impacting both mRNA representation and exon splicing. These data lead us to conclude that PCBP1 and PCBP2 mediate functions critical to differentiation of the erythroid lineage.

Selective effects of protein 4.1N deficiency on neuroendocrine and reproductive systems

Protein 4.1N, a member of the protein 4.1 family, is highly expressed in the brain. But its function remains to be fully defined. Using 4.1N^−/− mice, we explored the function of 4.1N in vivo. We show that 4.1N^−/− mice were born at a significantly reduced Mendelian ratio and exhibited high mortality between 3 to 5 weeks of age. Live 4.1N^−/− mice were smaller than 4.1N^+/+ mice. Notably, while there were no significant differences in organ/body weight ratio for most of the organs, the testis/body and ovary/body ratio were dramatically decreased in 4.1N^−/− mice, demonstrating selective effects of 4.1N deficiency on the development of the reproductive systems. Histopathology of the reproductive organs showed atrophy of both testis and ovary. Specifically, in the testis there is a lack of spermatogenesis, lack of leydig cells and lack of mature sperm. Similarly, in the ovary there is a lack of follicular development and lack of corpora lutea formation, as well as lack of secretory changes in the endometrium. Examination of pituitary glands revealed that the secretory granules were significantly decreased in pituitary glands of 4.1N^−/− compared to 4.1N^+/+. Moreover, while GnRH was expressed in both neuronal cell body and axons in the hypothalamus of 4.1N^+/+ mice, it was only expressed in the cell body but not the axons of 4.1N^-/- mice. Our findings uncover a novel role for 4.1N in the axis of hypothalamus-pituitary gland-reproductive system.

Cytoskeleton protein 4.1R regulates B-cell fate by modulating the canonical NF-κB pathway

During the immune response, B cells can enter the memory pathway, which is characterized by class switch recombination (CSR), or they may undergo plasma cell differentiation (PCD) to secrete immunoglobulin. Both of these processes occur in activated B cells, which are reported to relate to membrane-association proteins and adaptors. Protein 4.1R acts as an adaptor, linking membrane proteins to the cytoskeleton, and is involved in many cell events such as cell activation and differentiation, and cytokine secretion. However, the effect of 4.1R on regulating B-cell fate is unclear. Here, we show an important association between B-cell fate and 4.1R. In vitro, primary B cells were stimulated with lipopolysaccharide combined with interleukin-4; results showed that 4.1R-deficient (4.1R^-/- ) cells compared with wild-type (4.1R^+/+ ) B cells augmented expression of activation-induced cytidine deaminase and germline, resulting in increased IgG1⁺ B cells, whereas the secretion of IgG1 and IgM was reduced, and CD138⁺ B cells were also decreased. Throughout the process, 4.1R regulated canonical nuclear factor (NF-κB) rather than non-canonical NF-κB to promote the expression of CSR complex components, leading to up-regulation of B-cell CSR. In contrast, 4.1R-deficient B cells showed reduced expression of Blimp-1, which caused B cells to down-regulate PCD. Furthermore, over-activation of canonical NF-κB may induce apoptosis signaling to cause PCD apoptosis to reduce PCD number. In summary, our results suggest that 4.1R acts as a B-cell fate regulator by inhibiting the canonical NF-κB signaling pathway.

Abnormal expression and prognostic significance of EPB41L1 in kidney renal clear cell carcinoma based on data mining

Background

EPB41L1 gene (erythrocyte membrane protein band 4.1 like 1) encodes the protein 4.1N, a member of 4.1 family, playing a vital role in cell adhesion and migration, which is associated with the malignant progression of various human cancers. However, the expression and prognostic significance of EPB41L1 in kidney renal clear cell carcinoma (KIRC) remain to be investigated.

Methods

In this study, we collected the mRNA expression of EPB41L1 in KIRC through the Oncomine platform, and used the HPA database to perform the pathological tissue immunohistochemistry in patients. Then, the sub-groups and prognosis of KIRC were performed by UALCAN and GEPIA web-tool, respectively. Further, the mutation of EPB41L1 in KIRC was analyzed by c-Bioportal. The co-expression genes of EPB41L1 in KIRC were displayed from the LinkedOmics database, and function enrichment analysis was used by LinkFinder module in LinkedOmics. The function of EPB41L1 in cell adhesion and migration was confirmed by wound healing assay using 786-O cells in vitro. Co-expression gene network was constructed through the STRING database, and the MCODE plug-in of which was used to build the gene modules, both of them was visualized by Cytoscape software. Finally, the top modular genes in the same patient cohort were constructed through data mining in TCGA by using the UCSC Xena browser.

Results

The results indicated that EPB41L1 was down-expressed in KIRC, leading to a poor prognosis. Moreover, there is a mutation in the FERM domain of EPB41L1, but it has no significant effect on the prognosis of KIRC. The co-expressed genes of EPB41L1 were associated with cell adhesion and confirmed in vitro. Further analysis suggested that EPB41L1 and amyloid beta precursor protein (APP) were coordinated to regulated cancer cell adhesion, thereby increasing the incidence of cancer cell metastasis and tumor invasion.

Conclusions

In summary, EPB41L1 is constantly down-expressed in KIRC tissues, resulting a poor prognosis. Therefore, we suggest that it can be an effective biomarker for the diagnosis of KIRC.

Red Blood Cells as Potential Repositories of MicroRNAs in the Circulatory System

The amount of erythrocyte-derived microRNAs (miRNAs) represents the majority of miRNAs expressed in whole blood. miR-451, miR-144, and miR-486, which are abundant in red blood cells (RBCs), are involved in the process of erythropoiesis and disease occurrence. Moreover, erythrocyte-derived miRNAs have been reported to be potential biomarkers of specific diseases. However, the function and underlying mechanisms of miRNAs derived from erythrocytes remain unclear. Based on a review of previously published literature, we discuss several possible pathways by which RBC miRNAs may function and propose that RBCs may serve as repositories of miRNAs in the circulatory system and participate in the regulation of gene expression mainly via the transfer of miRNAs from erythrocyte extracellular vesicles (EVs). In the whole blood, there are still other important cell types such as leukocytes and platelets harboring functional miRNAs, and hemolysis also exists, which limit the abundance of miRNAs as disease biomarkers, and thus, miRNA studies on RBCs may be impacted. In the future, the role of RBCs in the regulation of normal physiological functions of the body and the entire circulatory system under pathological states, if any, remains to be determined.

Multiple environmental stressors induce complex transcriptomic responses indicative of phenotypic outcomes in Western fence lizard

BACKGROUND

The health and resilience of species in natural environments is increasingly challenged by complex anthropogenic stressor combinations including climate change, habitat encroachment, and chemical contamination. To better understand impacts of these stressors we examined the individual- and combined-stressor impacts of malaria infection, food limitation, and 2,4,6-trinitrotoluene (TNT) exposures on gene expression in livers of Western fence lizards (WFL, Sceloporus occidentalis) using custom WFL transcriptome-based microarrays.

RESULTS

Computational analysis including annotation enrichment and correlation analysis identified putative functional mechanisms linking transcript expression and toxicological phenotypes. TNT exposure increased transcript expression for genes involved in erythropoiesis, potentially in response to TNT-induced anemia and/or methemoglobinemia and caused dose-specific effects on genes involved in lipid and overall energy metabolism consistent with a hormesis response of growth stimulation at low doses and adverse decreases in lizard growth at high doses. Functional enrichment results were indicative of inhibited potential for lipid mobilization and catabolism in TNT exposures which corresponded with increased inguinal fat weights and was suggestive of a decreased overall energy budget. Malaria infection elicited enriched expression of multiple immune-related functions likely corresponding to increased white blood cell (WBC) counts. Food limitation alone enriched functions related to cellular energy production and decreased expression of immune responses consistent with a decrease in WBC levels.

CONCLUSIONS

Despite these findings, the lizards demonstrated immune resilience to malaria infection under food limitation with transcriptional results indicating a fully competent immune response to malaria, even under bio-energetic constraints. Interestingly, both TNT and malaria individually increased transcriptional expression of immune-related genes and increased overall WBC concentrations in blood; responses that were retained in the TNT x malaria combined exposure. The results demonstrate complex and sometimes unexpected responses to multiple stressors where the lizards displayed remarkable resiliency to the stressor combinations investigated.

Microfluidic assay of the deformability of primitive erythroblasts

Primitive erythroblasts (precursors of red blood cells) enter vascular circulation during the embryonic period and mature while circulating. As a result, primitive erythroblasts constantly experience significant hemodynamic shear stress. Shear-induced deformation of primitive erythroblasts however, is poorly studied. In this work, we examined the deformability of primitive erythroblasts at physiologically relevant flow conditions in microfluidic channels and identified the regulatory roles of the maturation stage of primitive erythroblasts and cytoskeletal protein 4.1 R in shear-induced cell deformation. The results showed that the maturation stage affected the deformability of primitive erythroblasts significantly and that primitive erythroblasts at later maturational stages exhibited a better deformability due to a matured cytoskeletal structure in the cell membrane.

4.1N suppresses hypoxia-induced epithelial-mesenchymal transition in epithelial ovarian cancer cells

Protein 4.1N (4.1N) is a member of the protein 4.1 family and is essential for the regulation of cell adhesion, motility and signaling. Previous studies have suggested that 4.1N may serve a tumor suppressor role. However, the molecular mechanisms remain unclear. In the current study, the role of 4.1N in the downregulation of hypoxia‑induced factor 1α (HIF‑1α) under hypoxic conditions and therefore the suppression of hypoxia induced epithelial‑mesenchymal transition (EMT) was investigated. The data were obtained from overexpressed and knockdown 4.1N epithelial ovarian cancer (EOC) cell lines. It was identified that 4.1N was capable of regulating the sub‑cellular localization and expression levels of HIF‑1α, by which 4.1N served a dominant role in the suppression of hypoxia‑induced EMT and associated genes. Collectively, the data of the current study identified 4.1N as an inhibitor of hypoxia‑induced tumor progression in EOC cells and highlighted its potential role in EOC therapy.

A comprehensive joint analysis of the long and short RNA transcriptomes of human erythrocytes

Background

Human erythrocytes are terminally differentiated, anucleate cells long thought to lack RNAs. However, previous studies have shown the persistence of many small-sized RNAs in erythrocytes. To comprehensively define the erythrocyte transcriptome, we used high-throughput sequencing to identify both short (18–24 nt) and long (>200 nt) RNAs in mature erythrocytes.

Results

Analysis of the short RNA transcriptome with miRDeep identified 287 known and 72 putative novel microRNAs. Unexpectedly, we also uncover an extensive repertoire of long erythrocyte RNAs that encode many proteins critical for erythrocyte differentiation and function. Additionally, the erythrocyte long RNA transcriptome is significantly enriched in the erythroid progenitor transcriptome. Joint analysis of both short and long RNAs identified several loci with co-expression of both microRNAs and long RNAs spanning microRNA precursor regions. Within the miR-144/451 locus previously implicated in erythroid development, we observed unique co-expression of several primate-specific noncoding RNAs, including a lncRNA, and miR-4732-5p/-3p. We show that miR-4732-3p targets both SMAD2 and SMAD4, two critical components of the TGF-β pathway implicated in erythropoiesis. Furthermore, miR-4732-3p represses SMAD2/4-dependent TGF-β signaling, thereby promoting cell proliferation during erythroid differentiation.

Conclusions

Our study presents the most extensive profiling of erythrocyte RNAs to date, and describes primate-specific interactions between the key modulator miR-4732-3p and TGF-β signaling during human erythropoiesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2156-2) contains supplementary material, which is available to authorized users.

The human Kell blood group binds the erythroid 4.1R protein: new insights into the 4.1R-dependent red cell membrane complex

Protein 4.1R plays an important role in maintaining the mechanical properties of the erythrocyte membrane. We analysed the expression of Kell blood group protein in erythrocytes from a patient with hereditary elliptocytosis associated with complete 4.1R deficiency (4.1(-) HE). Flow cytometry and Western blot analyses revealed a severe reduction of Kell. In vitro pull down and co-immunoprecipitation experiments from erythrocyte membranes showed a direct interaction between Kell and 4.1R. Using different recombinant domains of 4.1R and the cytoplasmic domain of Kell, we demonstrated that the R(46) R motif in the juxta-membrane region of Kell binds to lobe B of the 4.1R FERM domain. We also observed that 4.1R deficiency is associated with a reduction of XK and DARC (also termed ACKR1) proteins, the absence of the glycosylated form of the urea transporter B and a slight decrease of band 3. The functional alteration of the 4.1(-) HE erythrocyte membranes was also determined by measuring various transport activities. We documented a slower rate of HCO3 (-) /Cl(-) exchange, but normal water and ammonia transport across erythrocyte membrane in the absence of 4.1. These findings provide novel insights into the structural organization of blood group antigen proteins into the 4.1R complex of the human red cell membrane.

FERM family proteins and their importance in cellular movements and wound healing (review)

Motility is a requirement for a number of biological processes, including embryonic development, neuronal development, immune responses, cancer progression and wound healing. Specific to wound healing is the migration of endothelial cells, fibroblasts and other key cellular players into the wound space. Aberrations in wound healing can result in either chronic wounds or abnormally healed wounds. The protein 4.1R, ezrin, radixin, moesin (FERM) superfamily consists of over 40 proteins all containing a three lobed N-terminal FERM domain which binds a variety of cell-membrane associated proteins and lipids. The C-terminal ends of these proteins typically contain an actin-binding domain (ABD). These proteins therefore mediate the linkage between the cell membrane and the actin cytoskeleton, and are involved in cellular movements and migration. Certain FERM proteins have been shown to promote cancer metastasis via this very mechanism. Herein we review the effects of a number of FERM proteins on wound healing and cancer. We show how these proteins typically aid wound healing through their effects on increasing cellular migration and movements, but also typically promote metastasis in cancer. We conclude that FERM proteins play important roles in cellular migration, with markedly different outcomes in the context of cancer and wound healing.

The Protein 4.1 family: hub proteins in animals for organizing membrane proteins

Proteins of the 4.1 family are characteristic of eumetazoan organisms. Invertebrates contain single 4.1 genes and the Drosophila model suggests that 4.1 is essential for animal life. Vertebrates have four paralogues, known as 4.1R, 4.1N, 4.1G and 4.1B, which are additionally duplicated in the ray-finned fish. Protein 4.1R was the first to be discovered: it is a major mammalian erythrocyte cytoskeletal protein, essential to the mechanochemical properties of red cell membranes because it promotes the interaction between spectrin and actin in the membrane cytoskeleton. 4.1R also binds certain phospholipids and is required for the stable cell surface accumulation of a number of erythrocyte transmembrane proteins that span multiple functional classes; these include cell adhesion molecules, transporters and a chemokine receptor. The vertebrate 4.1 proteins are expressed in most tissues, and they are required for the correct cell surface accumulation of a very wide variety of membrane proteins including G-Protein coupled receptors, voltage-gated and ligand-gated channels, as well as the classes identified in erythrocytes. Indeed, such large numbers of protein interactions have been mapped for mammalian 4.1 proteins, most especially 4.1R, that it appears that they can act as hubs for membrane protein organization. The range of critical interactions of 4.1 proteins is reflected in disease relationships that include hereditary anaemias, tumour suppression, control of heartbeat and nervous system function. The 4.1 proteins are defined by their domain structure: apart from the spectrin/actin-binding domain they have FERM and FERM-adjacent domains and a unique C-terminal domain. Both the FERM and C-terminal domains can bind transmembrane proteins, thus they have the potential to be cross-linkers for membrane proteins. The activity of the FERM domain is subject to multiple modes of regulation via binding of regulatory ligands, phosphorylation of the FERM associated domain and differential mRNA splicing. Finally, the spectrum of interactions of the 4.1 proteins overlaps with that of another membrane-cytoskeleton linker, ankyrin. Both ankyrin and 4.1 link to the actin cytoskeleton via spectrin, and we hypothesize that differential regulation of 4.1 proteins and ankyrins allows highly selective control of cell surface protein accumulation and, hence, function. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

The RNA binding protein RBM38 (RNPC1) regulates splicing during late erythroid differentiation

Alternative pre-mRNA splicing is a prevalent mechanism in mammals that promotes proteomic diversity, including expression of cell-type specific protein isoforms. We characterized a role for RBM38 (RNPC1) in regulation of alternative splicing during late erythroid differentiation. We used an Affymetrix human exon junction (HJAY) splicing microarray to identify a panel of RBM38-regulated alternatively spliced transcripts. Using microarray databases, we noted high RBM38 expression levels in CD71⁺ erythroid cells and thus chose to examine RBM38 expression during erythroid differentiation of human hematopoietic stem cells, detecting enhanced RBM38 expression during late erythroid differentiation. In differentiated erythroid cells, we validated a subset of RBM38-regulated splicing events and determined that RBM38 regulates activation of Protein 4.1R (EPB41) exon 16 during late erythroid differentiation. Using Epb41 minigenes, Rbm38 was found to be a robust activator of exon 16 splicing. To further address the mechanism of RBM38-regulated alternative splicing, a novel mammalian protein expression system, followed by SELEX-Seq, was used to identify a GU-rich RBM38 binding motif. Lastly, using a tethering assay, we determined that RBM38 can directly activate splicing when recruited to a downstream intron. Together, our data support the role of RBM38 in regulating alternative splicing during erythroid differentiation.

Defective expression of Protein 4.1N is correlated to tumor progression, aggressive behaviors and chemotherapy resistance in epithelial ovarian cancer

OBJECTIVE

Protein 4.1N (4.1N) is a member of the Protein 4.1 family that is involved in cellular processes such as cell adhesion, migration and signaling. In this study, we evaluated the expression of 4.1N protein and its potential roles in epithelial ovarian cancer (EOC) tumorigenesis and progression.

METHODS

4.1N protein expression was investigated in a total of 280 samples including 74 normal tissues, 35 benign, 30 borderline and 141 malignant epithelial ovarian tumors by immunohistochemistry. Correlation between 4.1N expression levels and clinicopathologic features was statistically analyzed. The expression of 4.1N in EOC cell lines was examined by western blotting.

RESULTS

Immunohistochemistry analysis revealed that, although there was no loss of 4.1N expression in normal tissues and benign tumors, absence of Protein 4.1N was significantly more common in EOCs (44.0%) than in borderline tumors (3.3%) (p<0.001). Furthermore, loss or decreased expression of 4.1N protein expression was correlated with malignant potential of the tumors (14.3% in benign tumors, 56.7% in borderline tumors and 92.9% in malignancy) (p<0.001). In EOC samples, loss of 4.1N protein was significantly associated with advanced-stage (p=0.004), ascites (p=0.009), omental metastasis (p=0.018), suboptimal debulking (p=0.024), poorly histological differentiation (p=0.009), high-grade serous carcinoma (p=0.001), short progression-free-survival (p=0.018) and poor chemosensitivity to first-line chemotherapy (p=0.029). Moreover, western blotting analysis revealed that expression of 4.1N protein was lost in 4/8 (50%) EOC cell lines.

CONCLUSIONS

4.1N protein expression level was significantly decreased during malignant transformation of epithelial ovarian tumors and that loss of 4.1N expression was closely correlated to poorly differentiated and biologically aggressive EOCs.

The membrane-cytoskeletal protein 4.1N is involved in the process of cell adhesion, migration and invasion of breast cancer cells

Protein 4.1N belongs to the protein 4.1 superfamily that links transmembrane proteins to the actin cytoskeleton. Recent evidence has shown that protein 4.1 is important in tumor suppression. However, the functions of 4.1N in the metastasis of breast cancer are largely unknown. In the present study, MCF-7, T-47D and MDA-MB-231 breast cancer cell lines with various metastatic abilities were employed. Protein 4.1N was found to be expressed in poorly metastatic MCF-7 and middle metastatic T-47D cell lines, and was predominantly associated with cell-cell junctions. However, no 4.1N expression was detected in the highly metastatic MDA-MB-231 cells. Moreover, re-expression of 4.1N in MDA-MB-231 cells inhibited cell adhesion, migration and invasion. The results suggest that protein 4.1N is a negative regulator of cell metastasis in breast cancer.

Lack of protein 4.1G causes altered expression and localization of the cell adhesion molecule nectin-like 4 in testis and can cause male infertility

Protein 4.1G is a member of the protein 4.1 family, which in general serves as adaptors linking transmembrane proteins to the cytoskeleton. 4.1G is thought to be widely expressed in many cells and tissues, but its function remains largely unknown. To explore the function of 4.1G in vivo, we generated 4.1G(-/-) mice and bred the mice in two backgrounds: C57BL/6 (B6) and 129/Sv (129) hybrids (B6-129) and inbred B6. Although the B6 4.1G(-/-) mice showed no obvious abnormalities, deficiency of 4.1G in B6-129 hybrids was associated with male infertility. Histological examinations of these 4.1G(-/-) mice revealed atrophy, impaired cell-cell contact and sloughing off of spermatogenic cells in seminiferous epithelium, and lack of mature spermatids in the epididymis. Ultrastructural examination revealed enlarged intercellular spaces between spermatogenic and Sertoli cells as well as the spermatid deformities. At the molecular level, 4.1G is associated with the nectin-like 4 (NECL4) adhesion molecule. Importantly, the expression of NECL4 was decreased, and the localization of NECL4 was altered in 4.1G(-/-) testis. Thus, our findings imply that 4.1G plays a role in spermatogenesis by mediating cell-cell adhesion between spermatogenic and Sertoli cells through its interaction with NECL4 on Sertoli cells. Additionally, the finding that infertility is present in B6-129 but not on the B6 background suggests the presence of a major modifier gene(s) that influences 4.1G function and is associated with male infertility.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Protein 4.1R links E-cadherin/beta-catenin complex to the cytoskeleton through its direct interaction with beta-catenin and modulates adherens junction integrity

Protein 4.1R (4.1R) is the prototypical member of the protein 4.1 superfamily comprising of the protein 4.1 family (4.1R, 4.1B, 4.1G and 4.1N) and ERM family (ezrin, radixin and meosin). These proteins in general serve as adaptors between the membrane and the cytoskeleton. Here we show that 4.1R expressed in the gastric epithelial cells associates with adherens junction protein beta-catenin. Biochemical examination of 4.1R-deficient stomach epithelia revealed a selective reduction of beta-catenin which is accompanied by a weaker linkage of E-cadherin to the cytoskeleton. In addition, organization of actin cytoskeleton was altered in 4.1R-deficient cells. Moreover, histological examination revealed that cell-cell contacts are impaired and gastric glands are disorganized in 4.1R null stomach epithelia. These results demonstrate an important and previously unidentified role of 4.1R in linking the cadherin/catenin complex to the cytoskeleton through its direct interaction with beta-catenin and in regulating the integrity of adherens junction.

Cytoskeletal protein 4.1R negatively regulates T-cell activation by inhibiting the phosphorylation of LAT

Protein 4.1R (4.1R) was first identified in red cells where it plays an important role in maintaining mechanical stability of red cell membrane. 4.1R has also been shown to be expressed in T cells, but its function has been unclear. In the present study, we use 4.1R-deficient mice to explore the role of 4.1R in T cells. We show that 4.1R is recruited to the immunologic synapse after T cell-antigen receptor (TCR) stimulation. We show further that CD4+ T cells of 4.1R-/- mice are hyperactivated and that they displayed hyperproliferation and increased production of interleukin-2 (IL-2) and interferon gamma (IFNgamma). The hyperactivation results from enhanced phosphorylation of LAT and its downstream signaling molecule ERK. The 4.1R exerts its effect by binding directly to LAT, and thereby inhibiting its phosphorylation by ZAP-70. Moreover, mice deficient in 4.1R display an elevated humoral response to immunization with T cell-dependent antigen. Thus, we have defined a hitherto unrecognized role for 4.1R in negatively regulating T-cell activation by modulating intracellular signal transduction.

Purification of the NF2 tumor suppressor protein from human erythrocytes

BACKGROUND

Neurofibromatosis type 2 (NF2) is an autosomal dominant disease predisposing individuals to the risk of developing tumors of cranial and spinal nerves. The NF2 tumor suppressor protein, known as Merlin/Schwanomin, is a member of the protein 4.1 superfamily that function as links between the cytoskeleton and the plasma membrane.

METHODS

Upon selective extraction of membrane-associated proteins from erythrocyte plasma membrane (ghosts) using low ionic strength solution, the bulk of NF2 protein remains associated with the spectrin-actin depleted inside-out-vesicles. Western blot analysis showed a approximately 70 kDa polypeptide in the erythrocyte plasma membrane. Furthermore, quantitative removal of NF2 protein from the inside-out-vesicles was achieved using 1.0 M potassium iodide, a treatment known to remove tightly-bound peripheral membrane proteins.

RESULTS

These results suggest a novel mode of NF2 protein association with the erythrocyte membrane that is distinct from the known membrane interactions of protein 4.1. Based on these biochemical properties, several purification strategies were devised to isolate native NF2 protein from human erythrocyte ghosts. Using purified and recombinant NF2 protein as internal standards, we quantified approximately 41-65,000 molecules of NF2 protein per erythrocyte.

CONCLUSION

We provide evidence for the presence of NF2 protein in the human erythrocyte membrane. The identification of NF2 protein in the human erythrocyte membrane will make it feasible to discover novel interactions of NF2 protein utilizing powerful techniques of erythrocyte biochemistry and genetics in mammalian cells.

The 10 kDa domain of human erythrocyte protein 4.1 binds the Plasmodium falciparum EBA-181 protein

Background

Erythrocyte invasion by Plasmodium falciparum parasites represents a key mechanism during malaria pathogenesis. Erythrocyte binding antigen-181 (EBA-181) is an important invasion protein, which mediates a unique host cell entry pathway. A novel interaction between EBA-181 and human erythrocyte membrane protein 4.1 (4.1R) was recently demonstrated using phage display technology. In the current study, recombinant proteins were utilized to define and characterize the precise molecular interaction between the two proteins.

Methods

4.1R structural domains (30, 16, 10 and 22 kDa domain) and the 4.1R binding region in EBA-181 were synthesized in specific Escherichia coli strains as recombinant proteins and purified using magnetic bead technology. Recombinant proteins were subsequently used in blot-overlay and histidine pull-down assays to determine the binding domain in 4.1R.

Results

Blot overlay and histidine pull-down experiments revealed specific interaction between the 10 kDa domain of 4.1R and EBA-181. Binding was concentration dependent as well as saturable and was abolished by heat denaturation of 4.1R.

Conclusion

The interaction of EBA-181 with the highly conserved 10 kDa domain of 4.1R provides new insight into the molecular mechanisms utilized by P. falciparum during erythrocyte entry. The results highlight the potential multifunctional role of malaria invasion proteins, which may contribute to the success of the pathogenic stage of the parasite's life cycle.

Cardiac muscle cell cytoskeletal protein 4.1: Analysis of transcripts and subcellular location?relevance to membrane integrity, microstructure, and possible role in heart failure

The spectrin-based cytoskeleton assembly has emerged as a major player in heart functioning; however, cardiac protein 4.1, a key constituent, is uncharacterized. Protein 4.1 evolved to protect cell membranes against mechanical stresses and to organize membrane microstructure. 4.1 Proteins are multifunctional and, among other activities, link integral/signaling proteins on the plasma and internal membranes with the spectrin-based cytoskeleton. Four genes, EPB41, EPB41L1, EPB41L2, and EPB41L3 encode proteins 4.1R, 4.1N, 4.1G, and 4.1B, respectively. All are extensively spliced. Different isoforms are expressed according to tissue and developmental state, individual function being controlled through inclusion/exclusion of interactive domains. We have defined mouse and human cardiac 4.1 transcripts; other than 4. 1B in humans, all genes show activity. Cardiac transcripts constitutively include conserved FERM and C-terminal domains; both interact with membrane-bound signaling/transport/cell adhesion molecules. Variable splicing within and adjacent to the central spectrin/actin-binding domain enables regulation of cytoskeleton-binding activity. A novel heart-specific exon occurs in human 4.1G, but not in mouse. Immunofluorescence reveals 4.1 staining within mouse cardiomyocytes; thus, both at the plasma membrane and, interdigitated with sarcomeric myosin, across myofibrils in regions close to the sarcoplasmic reticulum. These are all regions to which spectrin locates. 4.1R in human heart shows similar distribution; however, there is limited plasma membrane staining. We conclude that cardiac 4.1s are highly regulated in their ability to crosslink plasma/integral cell membranes with the spectrin-actin cytoskeleton. We speculate that over the repetitive cycles of heart muscle contraction and relaxation, 4.1s are likely to locate, support, and coordinate functioning of key membrane-bound macromolecular assemblies.

The zebrafish band 4.1 member Mir is involved in cell movements associated with gastrulation

Cellular processes rely on dynamic events occurring between the cortical cytoskeleton and plasma membrane. Members of the Band 4.1 superfamily, which are best known for their ability to tether the cytoskeleton to the plasma membrane, play prominent structural and regulatory roles that influence cell-cell and cell-substrate interactions, endo- and exocytosis, cell polarity, migration, proliferation, and differentiation. We have identified a new member of the zebrafish Band 4.1 superfamily, which is the homolog of human myosin regulatory light chain interacting protein (MIR), and have examined its role in embryonic development. Zebrafish Mir contains the conserved amino-terminal plasma membrane-binding FERM (Band 4.1/ezrin/radixin/moesin) domain as well as other putative protein-protein interacting domains, including a RING finger. Overall, zebrafish Mir is 71% identical to human MIR located at chromosome 6p23-p22.3, and maps on linkage group 19 to a region of synteny with human chromosome 6. In situ hybridization and RT-PCR revealed that mir is expressed maternally and ubiquitously throughout development. Blocking Mir translation using a mir-specific, morpholino-based, knock-down strategy or expressing Mir constructs lacking the RING finger domain disrupts gastrulation and leads to subsequent trunk and tail defects. In severe cases, morphants exogastrulate. The synergistic effect seen when two mir-specific morpholinos are used in conjunction reflects the specific knock-down of mir. In addition, morphant phenotypes induced by mir-specific morpholinos are rescued by overexpression of the full-length Mir. In situ hybridization analysis with mesodermal- and neural-specific markers shows that morphants exhibit a delay in cell movements associated with gastrulation, epiboly, convergence, and extension. A yeast two-hybrid analysis was performed to identify binding partners that may participate with Mir during gastrulation, and Annexin V, a calcium channel protein, was isolated. At early developmental stages, annexin V transcripts colocalize with mir, but after gastrulation, annexin V mRNA becomes localized to the distal tail region and an area in the olfactory placode. At the protein level, Mir colocalizes with Annexin V when expressed in COS cells. Together, these results indicate that Mir is essential for embryonic development and that its role in early embryonic development likely involves calcium-dependent mechanisms essential during the extensive cell movements associated with gastrulation.

Surface expression of GluR-D AMPA receptor is dependent on an interaction between its C-terminal domain and a 4.1 protein

Dynamic regulation of the number and activity of AMPA receptors is believed to underlie many forms of synaptic plasticity and is presumably mediated by specific protein-protein interactions involving the C-terminal domain of the receptor. Several proteins interacting with the C-terminal tails of the glutamate receptor (GluR)-A and GluR-B subunits have been identified and implicated in the regulation of endocytosis and exocytosis, clustering, and anchoring of AMPA receptors to the cytoskeleton. In contrast, little is known of the molecular interactions of the GluR-D subunit, or of the mechanisms regulating the traffic of GluR-D-containing AMPA receptors. We analyzed the subcellular localization of homomeric GluR-D receptors carrying C-terminal deletions in transfected human embryonic kidney (HEK) 293 cells and in primary neurons by immunofluorescence microscopy and ELISA. A minimal requirement for a 14-residue cytoplasmic segment for the surface expression of homomeric GluR-D receptors was identified. Previously, a similar region in the GluR-A subunit was implicated in an interaction with 4.1 family proteins. Coimmunoprecipitation demonstrated that GluR-D associated with 4.1 protein(s) in both HEK293 cells and rat brain. Moreover, glutathione S-transferase pull-down experiments showed that the same 14-residue segment is critical for 4.1 binding to GluR-A and GluR-D. Point mutations within this segment dramatically decreased the surface expression of GluR-D in HEK293 cells, with a concomitant loss of the 4.1 interaction. Our findings demonstrate a novel molecular interaction for the GluR-D subunit and suggest that the association with the 4.1 family protein(s) plays an essential role in the transport to and stabilization of GluR-D-containing AMPA receptors at the cell surface.

Surface expression of GluR-D AMPA receptor is dependent on an interaction between its C-terminal domain and a 4.1 protein

Dynamic regulation of the number and activity of AMPA receptors is believed to underlie many forms of synaptic plasticity and is presumably mediated by specific protein-protein interactions involving the C-terminal domain of the receptor. Several proteins interacting with the C-terminal tails of the glutamate receptor (GluR)-A and GluR-B subunits have been identified and implicated in the regulation of endocytosis and exocytosis, clustering, and anchoring of AMPA receptors to the cytoskeleton. In contrast, little is known of the molecular interactions of the GluR-D subunit, or of the mechanisms regulating the traffic of GluR-D-containing AMPA receptors. We analyzed the subcellular localization of homomeric GluR-D receptors carrying C-terminal deletions in transfected human embryonic kidney (HEK) 293 cells and in primary neurons by immunofluorescence microscopy and ELISA. A minimal requirement for a 14-residue cytoplasmic segment for the surface expression of homomeric GluR-D receptors was identified. Previously, a similar region in the GluR-A subunit was implicated in an interaction with 4.1 family proteins. Coimmunoprecipitation demonstrated that GluR-D associated with 4.1 protein(s) in both HEK293 cells and rat brain. Moreover, glutathione S-transferase pull-down experiments showed that the same 14-residue segment is critical for 4.1 binding to GluR-A and GluR-D. Point mutations within this segment dramatically decreased the surface expression of GluR-D in HEK293 cells, with a concomitant loss of the 4.1 interaction. Our findings demonstrate a novel molecular interaction for the GluR-D subunit and suggest that the association with the 4.1 family protein(s) plays an essential role in the transport to and stabilization of GluR-D-containing AMPA receptors at the cell surface.

Characterization of zebrafish merlot/chablis as non-mammalian vertebrate models for severe congenital anemia due to protein 4.1 deficiency

The red blood cell membrane skeleton is an elaborate and organized network of structural proteins that interacts with the lipid bilayer and transmembrane proteins to maintain red blood cell morphology, membrane deformability and mechanical stability. A crucial component of red blood cell membrane skeleton is the erythroid specific protein 4.1R, which anchors the spectrin-actin based cytoskeleton to the plasma membrane. Qualitative and quantitative defects in protein 4.1R result in congenital red cell membrane disorders characterized by reduced cellular deformability and abnormal cell morphology. The zebrafish mutants merlot (mot) and chablis (cha) exhibit severe hemolytic anemia characterized by abnormal cell morphology and increased osmotic fragility. The phenotypic analysis of merlot indicates severe hemolysis of mutant red blood cells, consistent with the observed cardiomegaly, splenomegaly, elevated bilirubin levels and erythroid hyperplasia in the kidneys. The result of electron microscopic analysis demonstrates that mot red blood cells have membrane abnormalities and exhibit a severe loss of cortical membrane organization. Using positional cloning techniques and a candidate gene approach, we demonstrate that merlot and chablis are allelic and encode the zebrafish erythroid specific protein 4.1R. We show that mutant cDNAs from both alleles harbor nonsense point mutations, resulting in premature stop codons. This work presents merlot/chablis as the first characterized non-mammalian vertebrate models of hereditary anemia due to a defect in protein 4.1R integrity.

Plasmodium falciparum cysteine protease falcipain-2 cleaves erythrocyte membrane skeletal proteins at late stages of parasite development

Plasmodium falciparum-derived cysteine protease falcipain-2 cleaves host erythrocyte hemoglobin at acidic pH and specific components of the membrane skeleton at neutral pH. Analysis of stage-specific expression of these 2 proteolytic activities of falcipain-2 shows that hemoglobin-hydrolyzing activity is maximum in early trophozoites and declines rapidly at late stages, whereas the membrane skeletal protein hydrolyzing activity is markedly increased at the late trophozoite and schizont stages. Among the erythrocyte membrane skeletal proteins, ankyrin and protein 4.1 are cleaved by native and recombinant falcipain-2 near their C-termini. To identify the precise peptide sequence at the hydrolysis site of protein 4.1, we used a recombinant construct of protein 4.1 as substrate followed by MALDI-MS analysis of the cleaved product. We show that falcipain-2-mediated cleavage of protein 4.1 occurs immediately after lysine 437, which lies within a region of the spectrin-actin-binding domain critical for erythrocyte membrane stability. A 16-mer peptide containing the cleavage site completely inhibited the enzyme activity and blocked falcipain-2-induced fragmentation of erythrocyte ghosts. Based on these results, we propose that falcipain-2 cleaves hemoglobin in the acidic food vacuole at the early trophozoite stage, whereas it cleaves specific components of the red cell skeleton at the late trophozoite and schizont stages. It is the proteolysis of skeletal proteins that causes membrane instability, which, in turn, facilitates parasite release in vivo.

Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach

Membrane-bound receptors such as tyrosine kinases and ionotropic receptors are associated with large protein networks structured by protein-protein interactions involving multidomain proteins. Although these networks have emerged as a general mechanism of cellular signalling, much less is known about the protein complexes associated with G-protein-coupled receptors (GPCRs). Using a proteomic approach based on peptide affinity chromatography followed by mass spectrometry and immunoblotting, we have identified 15 proteins that interact with the C- terminal tail of the 5-hydroxytryptamine 2C (5-HT(2C)) receptor, a GPCR. These proteins include several synaptic multidomain proteins containing one or several PDZ domains (PSD95 and the proteins of the tripartite complex Veli3-CASK-Mint1), proteins of the actin/spectrin cytoskeleton and signalling proteins. Coimmunoprecipitation experiments showed that 5-HT(2C) receptors interact with PSD95 and the Veli3-CASK-Mint1 complex in vivo. Electron microscopy also indicated a synaptic enrichment of Veli3 and 5-HT(2C) receptors and their colocalization in microvilli of choroidal cells. These results indicate that the 5-HT(2C) receptor is associated with protein networks that are important for its synaptic localization and its coupling to the signalling machinery.

Properties of the C-terminal domain of 4.1 proteins

At the C-terminus of all known 4.1 proteins is a sequence domain unique to these proteins, known as the C-terminal domain (CTD). Mammalian CTDs are associated with a growing number of protein-protein interactions, although such activities have yet to be associated with invertebrate CTDs. Mammalian CTDs are generally defined by sequence alignment as encoded by exons 18-21. Comparison of known vertebrate 4.1 proteins with invertebrate (Caenorhabditis elegans and Drosophila melanogaster) 4.1 proteins indicates that mammalian 4.1 exon 19 represents a vertebrate adaptation that extends the sequence of the CTD with a Ser/Thr-rich sequence. The CTD was first described as a 22/24-kDa domain by chymotryptic digestion of erythrocyte 4.1 (4.1R) [Leto, T.L. & Marchesi, V.T. (1984) J. Biol. Chem. 259, 4603-4608]. Here we show that in 4.1R the 22/24-kDa fragment is not stable but rapidly processed to a 15-kDa fragment by chymotrypsin. The 15-kDa fragment is extremely stable, being resistant to overnight digestion in chymotrypsin on ice. Analysis of this fragment indicates that it is derived from residues 709-858 (SwissProt accession no. P48193), and represents the CTD of 4.1R. The fragment behaves as a globular monomer in solution. Secondary-structure predictions indicate that this domain is composed of five or six beta strands with an alpha helix before the most C-terminal of these. Together these data indicate that the CTD probably represents an independent folding structure which has gained function since the divergence of vertebrates from invertebrates.

Type II brain 4.1 (4.1B/KIAA0987), a member of the protein 4.1 family, is localized to neuronal paranodes

Histochemical analyses of type II brain 4.1/4.1B/KIAA0987, a member of the protein 4.1 family, were carried out in rat brain. In situ hybridization (ISH) showed that type II brain 4.1 mRNA is expressed in a variety of neuronal cells. In particular, type II brain 4.1 mRNA was actively transcribed in the cells of the mesencephalon and the brainstem, which have large myelinated nerve fibers. Expression of type II brain 4.1 mRNA was not observed at least in glial cells distributed in nerve fiber tracts. In immunohistochemical studies using anti-type II brain 4.1-specific antibody, the major immunosignals appeared as brilliant pairs of dots along nerve fibers. Such immunosignals were detected throughout the brain, but were highly concentrated in nerve fiber tracts. These data suggested that type II brain 4.1 is predominantly localized to neuronal paranodes. Detailed analysis concentrating on the nodal region indicated that type II brain 4.1 is present at the paranodal membrane but not in the axoplasm. Weaker type II brain 4.1-specific immunosignals were observed along the internodal membrane of myelinated axons and in the cytoplasm of some neuronal cells. Finally, comparative immunohistochemical studies using antibodies against the other three protein 4.1 family members, type I brain 4.1/4.1N/KIAA0338, erythroid type 4.1 (4.1R) and 4.1G, demonstrated that each of these proteins is distributed in a unique pattern in the cerebellum. Our results are the first to show that type II brain 4.1 is the only member of the protein 4.1 family localized to neuronal paranodes.

Cloning of the murine Krit1 cDNA reveals novel mammalian 5' coding exons

Human KRIT1 (Krev interaction trapped 1), a defective gene product in cerebral cavernous malformation, was cloned from a HeLa cell cDNA library by virtue of its interaction with Krev/rap1A, a small ras-family GTPase. We have now characterized the full-length cDNA for the murine orthologue that encodes a predicted protein of 736 amino acids, 207 amino acids longer than the previously reported human protein. 5' Rapid amplification of cDNA ends analysis of mouse mRNA demonstrated a single transcriptional start site. The putative initiator codon was found within a context that conformed well to the Kozak consensus sequence and was preceded by an in-frame termination codon. BLAST analysis revealed that conceptual translation of a fragment of human genomic DNA upstream of the 5' end of the reported KRIT1 coding sequence predicts extension of the human open reading frame by 207 codons with 95% amino acid identity between the novel putative human and murine amino termini. This block of coding sequence was divided among four exons that are flanked by consensus splice site sequences. The extreme evolutionary conservation of this region, including a putative nuclear localization signal, indicates functional importance. These data have immediate relevance to mutation screening efforts in cerebral cavernous malformation and may contribute to our understanding of the normal biology of KRIT1 and the pathogenesis of this disorder.

Molecular cloning and characterization of a protein tyrosine phosphatase enriched in testis, a putative murine homologue of human PTPMEG

Protein tyrosine phosphorylation is regulated by protein tyrosine kinase and protein tyrosine phosphatase activities. These two counteracting proteins are implicated in cell growth and transformation. Using polymerase chain reaction with degenerate primers, we have identified a novel mouse protein tyrosine phosphatase (PTP). This cDNA contains a single open reading frame of the predicted 926 amino acids. Those predicted amino acids showed significant identity with human megakaryocyte protein-tyrosine phosphatase by 91% in nucleotide sequences and 94% in amino acid sequences. We have identified that expression of this PTP is highly enriched in the testis in mouse and human and has been termed here as a 'testis-enriched phosphatase' (TEP). Northern analysis detected two mRNA species of 3.7 and 3.2kb for this PTP in mouse testis and the expression of TEP is regulated during development. The recombinant phosphatase domain possesses protein tyrosine phosphatase activity when expressed in Escherichia coli. Immunohistochemical analysis of the cellular localization of TEP on mouse testis sections showed that this PTP is specifically expressed in spermatocytes and spermatids within seminiferous tubules, suggesting an important role in spermatogenesis.

Isolation and characterization of human NBL4, a gene involved in the beta-catenin/tcf signaling pathway

beta-Catenin, a key regulator of cellular proliferation, is often mutated in various types of human cancer. To investigate cellular responses related to the beta-catenin signaling pathway, we applied a differential display method using mouse cells transfected with an activated form of mutant beta-catenin. This analysis and subsequent northern-blot hybridization confirmed that expression of a murine gene encoding NBL4 (novel band 4.1-like protein 4) was up-regulated by activation of beta-catenin. To examine a possible role of NBL4 in cancer, we isolated the human homologue of the murine NBL4 gene by matching mNBL4 against the human EST (expressed sequence tag) database followed by 5' rapid amplification of cDNA ends (5'RACE). The cDNA of hNBL4 encoded a protein of 598 amino acids that shared 87% identity in amino acid sequence with murine NBL4 and 71% with zebrafish NBL4. A 2.2-kb hNBL4 transcript was expressed in all human tissues examined with high levels of expression in brain, liver, thymus and peripheral blood leukocytes and low levels of expression in heart, kidney, testis and colon. We determined its chromosomal localization at 5q22 by fluorescence in situ hybridization. Expression of hNBL4 was significantly reduced when beta-catenin was depleted in SW480 cells, a human cancer cell line that constitutionally accumulates beta-catenin. The results support the view that NBL4 is an important component of the beta-catenin / Tcf pathway and is probably related to determination of cell polarity or proliferation.

Comparison of mRNA and protein levels of four members of the protein 4.1 family: the type II brain 4.1/4.1B/KIAA0987 is the most predominant member of the protein 4.1 family in rat brain

The human gene for the fourth member of the protein 4.1 family, KIAA0987, was recently identified by comprehensive cDNA analysis. To further characterize the corresponding gene and its product in rats, we cloned and sequenced rat KIAA0987 cDNA. RNA blot analyses revealed that the rat KIAA0987 gene was abundantly expressed only in the brain, kidney, and testis. Although we have previously reported that the third member of the protein 4.1 family, the KIAA0338 gene product, is predominantly expressed in rat brain, and thus was named brain 4.1, quantitative RNA blot analyses indicated that KIAA0987 should be called something other than brain 4.1 because the level of KIAA0987 mRNA was found to be of the same order as that of KIAA0338 mRNA. Our quantitative immunoblot analysis showed that the most predominant member of the protein 4.1 family at the protein level was the product of the KIAA0987 gene, not that of the KIAA0338 gene. Taking these results together, we consider it reasonable to name the KIAA0338 and KIAA0987 gene products 'type I brain 4.1' and 'type II brain 4.1,' respectively, because these two products were found to be more prominently produced in rat brain than the other two members of the protein 4.1 family, erythroid 4.1 and 4.1G.

Elliptocytosis in patients with C-terminal domain mutations of protein 4.1 correlates with encoded messenger RNA levels rather than with alterations in primary protein structure

Early biochemical studies defined 4 functional domains of the erythroid protein 4.1 (4.1R). From amino-terminal to carboxy-terminal, these are 30 kd, 16 kd, 10 kd, and 22/24 kd in size. Although the functional properties of both the 30-kd and the 10-kd domain have been demonstrated in red cells, no functional activities have been assigned to either the 16-kd or the 22/24-kd domain in these cells. We here describe new mutations in the sequence encoding the C-terminal 22/24-kd domain that are associated with hereditary elliptocytosis. An unusually mild phenotype observed in heterozygous and homozygous members of 1 family suggested heterogeneity in the pattern of expression of 4.1R deficiency. Using a variety of protein and messenger RNA (mRNA) quantification strategies, we showed that, regardless of the alteration in the C-terminal primary sequence, when the protein is produced, it assembles at the cell membrane. In addition, we found that alterations in red cell morphologic features and membrane function correlate with the amount of membrane-associated protein—and therefore with the amount of mRNA accumulated—rather than with the primary structure of the variant proteins. These data suggest that an intact sequence at exons 19 through 21 encoding part of the C-terminal 22/24-kd region is not required for proper protein 4.1R assembly in mature red cells.

What do mouse gene knockouts tell us about the structure and function of the red cell membrane?

Recent development of knockout mice with targeted deletion of specific genes encoding various red cell membrane proteins has added valuable armamentarium to red cell membrane structure-function studies. In this chapter we will summarize the various recent developments regarding the structure and function of the red cell membrane derived from studies using knockout mice. In addition to being expressed in red cells, all major red cell membrane proteins are also expressed in cells of various tissues. The potential use of knockout mice to decipher the biological functions of red cell membrane proteins in non-erythroid cells is also explored.

Cellular and subcellular localization of a newly identified member of the protein 4.1 family, brain 4.1, in the cerebellum of adult and postnatally developing rats

For obtaining a deeper insight into the properties of a newly characterized member of the protein 4.1 family, brain 4.1, the cellular and subcellular localization was investigated in the cerebellar cortex of adult and postnatally developing rats. Fluorescent immunohistochemical observations showed that brain 4.1 localized predominantly to glomeruli in the granular layer and throughout the molecular layer in adult rat cerebellar cortex. Analysis of subcellular localization of brain 4.1 by immuno-electron microscopy further demonstrated that presynaptic terminals of mossy fibers and parallel fibers, cytoplasm of granule cells and cytoplasm and/or processes of glial cells contained brain 4.1 while postsynaptic regions of the dendrites of granule cells and Purkinje cells, axons and myelin sheaths did not. Thus, one of the major subcellular destination of brain 4.1 was presynaptic terminal in the cerebellum. This was further supported by the fact that the immunostaining pattern of brain 4.1 in the cerebellum changed in a similar way to that of a synaptic terminal marker, synaptophysin during the postnatal development. Immunoblot analysis also demonstrated that contents of brain 4.1 isoforms varied in parallel with the changes of the immunostaining pattern. Biochemical analysis confirmed the presence of brain 4.1 at synaptic terminals, but there was no obvious correlation between each isoform and its subcellular localization. These results suggested that brain 4.1 is involved in the formation and maintenance of synapse as a membrane skeletal component at presynaptic terminals in the cerebellum.

A novel neuron-enriched homolog of the erythrocyte membrane cytoskeletal protein 4.1

We report the molecular cloning and characterization of 4.1N, a novel neuronal homolog of the erythrocyte membrane cytoskeletal protein 4.1 (4.1R). The 879 amino acid protein shares 70, 36, and 46% identity with 4.1R in the defined membrane-binding, spectrin-actin-binding, and C-terminal domains, respectively. 4.1N is expressed in almost all central and peripheral neurons of the body and is detected in embryonic neurons at the earliest stage of postmitotic differentiation. Like 4.1R, 4.1N has multiple splice forms as evidenced by PCR and Western analysis. Whereas the predominant 4.1N isoform identified in brain is approximately 135 kDa, a smaller 100 kDa isoform is enriched in peripheral tissues. Immunohistochemical studies using a polyclonal 4.1N antibody revealed several patterns of neuronal staining, with localizations in the neuronal cell body, dendrites, and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern was observed consistent with a synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at the discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (a postsynaptic marker) and glutamate receptor type 1 (an excitatory postsynaptic marker). By analogy with the roles of 4.1R in red blood cells, 4.1N may function to confer stability and plasticity to the neuronal membrane via interactions with multiple binding partners, including the spectrin-actin-based cytoskeleton, integral membrane channels and receptors, and membrane-associated guanylate kinases.

A novel neuron-enriched homolog of the erythrocyte membrane cytoskeletal protein 4.1

We report the molecular cloning and characterization of 4.1N, a novel neuronal homolog of the erythrocyte membrane cytoskeletal protein 4.1 (4.1R). The 879 amino acid protein shares 70, 36, and 46% identity with 4.1R in the defined membrane-binding, spectrin-actin-binding, and C-terminal domains, respectively. 4.1N is expressed in almost all central and peripheral neurons of the body and is detected in embryonic neurons at the earliest stage of postmitotic differentiation. Like 4.1R, 4.1N has multiple splice forms as evidenced by PCR and Western analysis. Whereas the predominant 4.1N isoform identified in brain is approximately 135 kDa, a smaller 100 kDa isoform is enriched in peripheral tissues. Immunohistochemical studies using a polyclonal 4.1N antibody revealed several patterns of neuronal staining, with localizations in the neuronal cell body, dendrites, and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern was observed consistent with a synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at the discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (a postsynaptic marker) and glutamate receptor type 1 (an excitatory postsynaptic marker). By analogy with the roles of 4.1R in red blood cells, 4.1N may function to confer stability and plasticity to the neuronal membrane via interactions with multiple binding partners, including the spectrin-actin-based cytoskeleton, integral membrane channels and receptors, and membrane-associated guanylate kinases.

A unique talin homologue with a villin headpiece-like domain is required for multicellular morphogenesis in Dictyostelium

Molecules involved in the interaction between the extracellular matrix, cell membrane and cytoskeleton are of central importance in morphogenesis. Talin is a large cytoskeletal protein with a modular structure consisting of an amino-terminal membrane-interacting domain, with sequence similarities to members of the band 4.1 family, and a carboxy-terminal region containing F-actin-binding and vinculin-binding domains [1] [2]. It also interacts with the cytoplasmic tail of beta integrins which, on the external face of the membrane, bind to extracellular matrix proteins [3]. The possible roles of talin in multicellular morphogenesis in development remain largely unexplored. In Dictyostelium, a eukaryotic microorganism capable of multicellular morphogenesis, a talin homologue (TALA) has previously been identified and shown to play an important role in cell-to-substrate adhesion and maintenance of normal elastic properties of the cell [4] [5] [6]. Here, we describe a second talin homologue (TALB) that is required for multicellular morphogenesis in the development of Dictyostelium. Unlike any other talin characterised to date, it contains an additional carboxy-terminal domain homologous to the villin headpiece.

Protein 4.1R-deficient mice are viable but have erythroid membrane skeleton abnormalities

A diverse family of protein 4.1R isoforms is encoded by a complex gene on human chromosome 1. Although the prototypical 80-kDa 4.1R in mature erythrocytes is a key component of the erythroid membrane skeleton that regulates erythrocyte morphology and mechanical stability, little is known about 4.1R function in nucleated cells. Using gene knockout technology, we have generated mice with complete deficiency of all 4.1R protein isoforms. These 4.1R-null mice were viable, with moderate hemolytic anemia but no gross abnormalities. Erythrocytes from these mice exhibited abnormal morphology, lowered membrane stability, and reduced expression of other skeletal proteins including spectrin and ankyrin, suggesting that loss of 4. 1R compromises membrane skeleton assembly in erythroid progenitors. Platelet morphology and function were essentially normal, indicating that 4.1R deficiency may have less impact on other hematopoietic lineages. Nonerythroid 4.1R expression patterns, viewed using histochemical staining for lacZ reporter activity incorporated into the targeted gene, revealed focal expression in specific neurons in the brain and in select cells of other major organs, challenging the view that 4.1R expression is widespread among nonerythroid cells. The 4.1R knockout mice represent a valuable animal model for exploring 4.1R function in nonerythroid cells and for determining pathophysiological sequelae to 4.1R deficiency.

An alternative domain determines nuclear localization in multifunctional protein 4.1

Multiple protein 4.1 isoforms are originated by alternative pre-mRNA splicing, differential use of two translation initiation sites, and posttranslational modifications. The complexity of alternative splicing events suffered by the 4.1 pre-mRNA makes necessary the direct cloning of 4.1 full-coding cDNA sequences to ensure that the encoded 4.1 proteins are naturally occurring isoforms. We have approached this point by reverse transcription-polymerase chain reaction techniques using RNA from the nucleated human Molt-4 T-cell line as a starting template. Molecular cloning of 4.1 cDNAs using the second translation initiation codon has allowed us to identify two 4.1 isoforms, designated 4.1H and 4.1I, which are differentially targeted to the nucleus (4.1H) and the cytoplasm (4.1I). These two isoforms differ only in the inclusion (4.1H) or exclusion (4.1I) of 21 amino acids encoded by exon 16. A cluster of basic amino acids, KKKR, generated by joining of the sequences encoded by the constitutive exon 13 and the alternative exon 16, is necessary for the nuclear targeting of 4.1H, as demonstrated by site-directed mutagenesis analysis. Immunofluorescence microscopy and biochemical studies indicate that 4.1H belongs to the group of nuclear 4.1 proteins that are distributed diffusely throughout the nucleoplasm and that are extractable in 0.5% Triton X-100. This is the first demonstration of differential nuclear targeting by the presence of an alternative domain, among naturally occurring protein 4.1 isoforms.

The 13-kD FK506 binding protein, FKBP13, interacts with a novel homologue of the erythrocyte membrane cytoskeletal protein 4.1

We have identified a novel generally expressed homologue of the erythrocyte membrane cytoskeletal protein 4.1, named 4.1G, based on the interaction of its COOH-terminal domain (CTD) with the immunophilin FKBP13. The 129-amino acid peptide, designated 4.1G-CTD, is the first known physiologic binding target of FKBP13. FKBP13 is a 13-kD protein originally identified by its high affinity binding to the immunosuppressant drugs FK506 and rapamycin (Jin, Y., M.W. Albers, W.S. Lane, B.E. Bierer, and S.J. Burakoff. 1991. Proc. Natl. Acad. Sci. USA. 88:6677- 6681); it is a membrane-associated protein thought to function as an ER chaperone (Bush, K.T., B.A. Henrickson, and S.K. Nigam. 1994. Biochem. J. [Tokyo]. 303:705-708). We report the specific association of FKBP13 with 4.1G-CTD based on yeast two-hybrid, in vitro binding and coimmunoprecipitation experiments. The histidyl-proline moiety of 4.1G-CTD is required for FKBP13 binding, as indicated by yeast experiments with truncated and mutated 4.1G-CTD constructs. In situ hybridization studies reveal cellular colocalizations for FKBP13 and 4.1G-CTD throughout the body during development, supporting a physiologic role for the interaction. Interestingly, FKBP13 cofractionates with the red blood cell homologue of 4.1 (4.1R) in ghosts, inside-out vesicles, and Triton shell preparations. The identification of FKBP13 in erythrocytes, which lack ER, suggests that FKBP13 may additionally function as a component of membrane cytoskeletal scaffolds.