Sequence analysis and chromosomal localization of human Cap Z. Conserved residues within the actin-binding domain may link Cap Z to gelsolin/severin and profilin protein families

Cell type-specific regulation of CFTR trafficking-on the verge of progress

Trafficking of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein is a complex process that starts with its biosynthesis and folding in the endoplasmic reticulum. Exit from the endoplasmic reticulum (ER) is coupled with the acquisition of a compact structure that can be processed and traffic through the secretory pathway. Once reaching its final destination-the plasma membrane, CFTR stability is regulated through interaction with multiple protein partners that are involved in its post-translation modification, connecting the channel to several signaling pathways. The complexity of the process is further boosted when analyzed in the context of the airway epithelium. Recent advances have characterized in detail the different cell types that compose the surface epithelium and shifted the paradigm on which cells express CFTR and on their individual and combined contribution to the total expression (and function) of this chloride/bicarbonate channel. Here we review CFTR trafficking and its relationship with the knowledge on the different cell types of the airway epithelia. We explore the crosstalk between these two areas and discuss what is still to be clarified and how this can be used to develop more targeted therapies for CF.

Genetics of BAG3: A Paradigm for Developing Precision Therapies for Dilated Cardiomyopathies

Nonischemic dilated cardiomyopathy is a common form of heart muscle disease in which genetic factors play a critical etiological role. In this regard, both rare disease-causing mutations and common disease-susceptible variants, in the Bcl-2-associated athanogene 3 (BAG3) gene have been reported, highlighting the critical role of BAG3 in cardiomyocytes and in the development of dilated cardiomyopathy. The phenotypic effects of the BAG3 mutations help investigators understand the structure and function of the BAG3 gene. Indeed, we report herein that all of the known pathogenic/likely pathogenic variants affect at least 1 of 3 protein functional domains, ie, the WW domain, the second IPV (Ile-Pro-Val) domain, or the BAG domain, whereas none of the missense nontruncating pathogenic/likely pathogenic variants affect the proline-rich repeat (PXXP) domain. A common variant, p.Cys151Arg, associated with reduced susceptibility to dilated cardiomyopathy demonstrated a significant difference in allele frequencies among diverse human populations, suggesting evolutionary selective pressure. As BAG3-related therapies for heart failure move from the laboratory to the clinic, the ability to provide precision medicine will depend in large part on having a thorough understanding of the potential effects of both common and uncommon genetic variants on these target proteins. The current review article provides a roadmap that investigators can utilize to determine the potential interactions between a patient's genotype, their phenotype, and their response to therapeutic interventions with both gene delivery and small molecules.

Genome-wide association study between copy number variation regions and carcass- and meat-quality traits in Nellore cattle

Context Indicine breeds are the main source of beef products in tropical and subtropical regions. However, genetic improvement for carcass- and meat-quality traits in zebu cattle have been limited and genomics studies concerning structural variations that influence these traits are essential. Aim The aim of this study was to perform a genome-wide association study between copy number variation regions (CNVRs) and carcass- and meat quality-traits in Nellore cattle. Methods In total, 3794 animals, males and females included, were genotyped using a 777962 single-nucleotide polymorphism platform of BovineHD BeadChip (777k; Illumina Inc.). Of these, 1751 Nellore bulls were slaughtered at 24 months of age for further carcass beef analysis. The following traits were studied: beef tenderness, marbling, rib-eye area, backfat thickness and meat colour (lightness, redness and yellowness). The CNV detection was conducted through PennCNV software. The association analyses were performed using CNVRuler software. Key results Several identified genomic regions were linked to quantitative trait loci associated with fat deposition (FABP7) and lipid metabolism (PPARA; PLA2 family; BCHE), extracellular matrix (INS; COL10A1), contraction (SLC34A3; TRDN) and muscle development (CAPZP). The gene-enrichment analyses highlighted biological mechanisms directly related to the metabolism and synthesis of lipids and fatty acids. Conclusions The large number of potential candidate genes identified within the CNVRs, as well as the functions and pathways identified, should help better elucidate the genetic mechanisms involved in the expression of beef and carcass traits in Nellore cattle. Several CNVRs harboured genes that might have a functional impact to improve the beef and carcass traits. Implications The results obtained contribute to upgrade the sensorial and organoleptic attributes of Nellore cattle and make feasible the genetic improvement of carcass- and meat-quality traits.

Cytoskeleton regulators CAPZA2 and INF2 associate with CFTR to control its plasma membrane levels under EPAC1 activation

Cystic Fibrosis (CF), the most common lethal autosomic recessive disorder among Caucasians, is caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein, a cAMP-regulated chloride channel expressed at the apical surface of epithelial cells. Cyclic AMP regulates both CFTR channel gating through a protein kinase A (PKA)-dependent process and plasma membane (PM) stability through activation of the exchange protein directly activated by cAMP1 (EPAC1). This cAMP effector, when activated promotes the NHERF1:CFTR interaction leading to an increase in CFTR at the PM by decreasing its endocytosis. Here, we used protein interaction profiling and bioinformatic analysis to identify proteins that interact with CFTR under EPAC1 activation as possible regulators of this CFTR PM anchoring. We identified an enrichment in cytoskeleton related proteins among which we characterized CAPZA2 and INF2 as regulators of CFTR trafficking to the PM. We found that CAPZA2 promotes wt-CFTR trafficking under EPAC1 activation at the PM whereas reduction of INF2 levels leads to a similar trafficking promotion effect. These results suggest that CAPZA2 is a positive regulator and INF2 a negative one for the increase of CFTR at the PM after an increase of cAMP and concomitant EPAC1 activation. Identifying the specific interactions involving CFTR and elicited by EPAC1 activation provides novel insights into late CFTR trafficking, insertion and/or stabilization at the PM and highlighs new potential therapeutic targets to tackle CF disease.

Functional homo- and heterodimeric actin capping proteins from the malaria parasite

Actin capping proteins belong to the core set of proteins minimally required for actin-based motility and are present in virtually all eukaryotic cells. They bind to the fast-growing barbed end of an actin filament, preventing addition and loss of monomers, thus restricting growth to the slow-growing pointed end. Actin capping proteins are usually heterodimers of two subunits. The Plasmodium orthologs are an exception, as their α subunits are able to form homodimers. We show here that, while the β subunit alone is unstable, the α subunit of the Plasmodium actin capping protein forms functional homo- and heterodimers. This implies independent functions for the αα homo- and αβ heterodimers in certain stages of the parasite life cycle. Structurally, the homodimers resemble canonical αβ heterodimers, although certain rearrangements at the interface must be required. Both homo- and heterodimers bind to actin filaments in a roughly equimolar ratio, indicating they may also bind other sites than barbed ends.

Systematic characterization of human testis-specific actin capping protein β3 as a possible biomarker for male infertility

STUDY QUESTION

Is actin capping protein (CP) β3 involved in human spermatogenesis and male infertility?

SUMMARY ANSWER

Human CPβ3 (hCPβ3) is expressed in testis, changes its localization dynamically during spermatogenesis, and has some association with male infertility.

WHAT IS KNOWN ALREADY

The testis-specific α subunit of CP (CPα3) was previously identified in human, and mutations in the cpα3 gene in mouse were shown to induce malformation of the sperm head and male infertility. However, CPβ3, which is considered to be a heterodimeric counterpart of CPα3, has been neither characterized in human nor reported in association with male infertility.

STUDY DESIGN, SIZE, DURATION

To confirm the existence of CPβ3 in human testis, fresh semen samples from proven fertile men were analyzed. To investigate protein expression during spermatogenesis, cryopreserved testis obtained from men with obstructive azoospermia were examined by immunofluorescent analysis. To assess the association of CP with male infertility, we compared protein expression of human CPα3 (hCPα3) and hCPβ3 using immunofluorescent analysis of cryopreserved sperm between men with normozoospermia (volunteers: Normo group, n = 20) and infertile men with oligozoospermia and/or asthenozoospermia (O + A group, n = 21).

PARTICIPANTS/MATERIALS, SETTING, METHODS

The tissue-specific expression of hCPβ3 was investigated by RT-PCR and Western blot analysis. To investigate whether hCPα3 and hCPβ3 form a heterodimer, a tandem expression vector containing hcpα3 tagged with monomeric red fluorescent protein 1 and hcpβ3 tagged with enhanced green fluorescent protein in a single plasmid was constructed and analyzed by co-immunoprecipitation (Co-IP) assay. The protein expression profiles of hCPα3 and hCPβ3 during spermatogenesis were examined by immunohistochemical analysis using human spermatogenic cells. The protein expressions of hCPα3 and hCPβ3 in sperm were compared between the Normo and O + A groups by immunohistochemical analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

RT-PCR showed that mRNA of hcpβ3 was expressed exclusively in testis. Western blot analysis detected hCPβ3 with anti-bovine CPβ3 antibody. Co-IP assay with recombinant protein showed that hCPα3 and hCPβ3 form a protein complex. At each step during spermatogenesis, the cellular localization of hCPβ3 changed dynamically. In spermatogonia, hCPβ3 showed a slight signal in cytoplasm. hCPβ3 expression was conspicuous mainly from spermatocytes, and hCPβ3 localization dynamically migrated from cytoplasm to the acrosomal cap and acrosome. In mature spermatozoa, hCPβ3 accumulated in the postacrosomal region and less so at the midpiece of the tail. Double-staining analysis revealed that hCPα3 localization was identical to hCPβ3 at every step in the spermatogenic cells. Most spermatozoa from the Normo group were stained homogenously by both hCPα3 and hCPβ3. In contrast, significantly more spermatozoa in the O + A versus Normo group showed heterogeneous or lack of staining for either hCPα3 or hCPβ3 (abnormal staining) (P < 0.001). The percentage of abnormal staining was higher in the O + A group (52.4 ± 3.0%) than in the Normo group (31.2 ± 2.5%). Even by confining the observations to morphologically normal spermatozoa selected in accordance with David's criteria, the percentage of abnormal staining was still higher in the O + A group (39.9 ± 2.9%) versus the Normo group (22.5 ± 2.1%) (P < 0.001). hCPβ3 in conjunction with hCPα3 seemed to play an important role in spermatogenesis and may be associated with male infertility.

LARGE SCALE DATA

Not applicable.

LIMITATIONS REASONS FOR CAUTION

Owing to the difficulty of collecting fresh samples of human testis, we used cryopreserved samples from testicular sperm extraction. To examine the interaction of spermatogenic cells or localization in seminiferous tubules, fresh testis sample of healthy males are ideal.

WIDER IMPLICATIONS OF THE FINDINGS

The altered expression of hCPα3 and hCPβ3 may not only be a cause of male infertility but also a prognostic factor for the results of ART. They may be useful biomarkers to determine the fertilization ability of human sperm in ART.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by a Grant-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science (JP16K20133). The authors declare no competing interests.

Tankyrase-Binding Protein TNKS1BP1 Regulates Actin Cytoskeleton Rearrangement and Cancer Cell Invasion

Tankyrase, a PARP that promotes telomere elongation and Wnt/β-catenin signaling, has various binding partners, suggesting that it has as-yet unidentified functions. Here, we report that the tankyrase-binding protein TNKS1BP1 regulates actin cytoskeleton and cancer cell invasion, which is closely associated with cancer progression. TNKS1BP1 colocalized with actin filaments and negatively regulated cell invasion. In TNKS1BP1-depleted cells, actin filament dynamics, focal adhesion, and lamellipodia ruffling were increased with activation of the ROCK/LIMK/cofilin pathway. TNKS1BP1 bound the actin-capping protein CapZA2. TNKS1BP1 depletion dissociated CapZA2 from the cytoskeleton, leading to cofilin phosphorylation and enhanced cell invasion. Tankyrase overexpression increased cofilin phosphorylation, dissociated CapZA2 from cytoskeleton, and enhanced cell invasion in a PARP activity-dependent manner. In clinical samples of pancreatic cancer, TNKS1BP1 expression was reduced in invasive regions. We propose that the tankyrase-TNKS1BP1 axis constitutes a posttranslational modulator of cell invasion whose aberration promotes cancer malignancy. Cancer Res; 77(9); 2328-38. ©2017 AACR.

Polycystin-2 (TRPP2) Regulation by Ca(2+) Is Effected and Diversified by Actin-Binding Proteins

Calcium regulation of Ca(2+)-permeable ion channels is an important mechanism in the control of cell function. Polycystin-2 (PC2, TRPP2), a member of the transient receptor potential superfamily, is a nonselective cation channel with Ca(2+) permeability. The molecular mechanisms associated with PC2 regulation by Ca(2+) remain ill-defined. We recently demonstrated that PC2 from human syncytiotrophoblast (PC2hst) but not the in vitro translated protein (PC2(iv)), functionally responds to changes in intracellular (cis) Ca(2+). In this study we determined the regulatory effect(s) of Ca(2+)-sensitive and -insensitive actin-binding proteins (ABPs) on PC2(iv) channel function in a lipid bilayer system. The actin-bundling protein α-actinin increased PC2(iv) channel function in the presence of cis Ca(2+), although instead was inhibitory in its absence. Conversely, filamin that shares actin-binding domains with α-actinin had a strong inhibitory effect on PC2(iv) channel function in the presence, but no effect in the absence of cis Ca(2+). Gelsolin stimulated PC2(iv) channel function in the presence, but not the absence of cis Ca(2+). In contrast, profilin that shares actin-binding domains with gelsolin, significantly increased PC2(iv) channel function both in the presence and absence of Ca(2+). The distinct effect(s) of the ABPs on PC2(iv) channel function demonstrate that Ca(2+) regulation of PC2 is actually mediated by direct interaction(s) with structural elements of the actin cytoskeleton. These data indicate that specific ABP-PC2 complexes would confer distinct Ca(2+)-sensitive properties to the channel providing functional diversity to the cytoskeletal control of transient receptor potential channel regulation.

Pivotal and distinct role for Plasmodium actin capping protein alpha during blood infection of the malaria parasite

Accurate regulation of microfilament dynamics is central to cell growth, motility and response to environmental stimuli. Stabilizing and depolymerizing proteins control the steady-state levels of filamentous (F-) actin. Capping protein (CP) binds to free barbed ends, thereby arresting microfilament growth and restraining elongation to remaining free barbed ends. In all CPs characterized to date, alpha and beta subunits form the active heterodimer. Here, we show in a eukaryotic parasitic cell that the two CP subunits can be functionally separated. Unlike the beta subunit, the CP alpha subunit of the apicomplexan parasite Plasmodium is refractory to targeted gene deletion during blood infection in the mammalian host. Combinatorial complementation of Plasmodium berghei CP genes with the orthologs from Plasmodium falciparum verified distinct activities of CP alpha and CP alpha/beta during parasite life cycle progression. Recombinant Plasmodium CP alpha could be produced in Escherichia coli in the absence of the beta subunit and the protein displayed F-actin capping activity. Thus, the functional separation of two CP subunits in a parasitic eukaryotic cell and the F-actin capping activity of CP alpha expand the repertoire of microfilament regulatory mechanisms assigned to CPs.

Identification and characterization of a differentially expressed protein (CAPZB) in skeletal muscle between Meishan and Large White pigs

Actin capping protein beta (CAPZB) protein was identified with considerable differences in the longissimus dorsi muscle between Large White and Meishan pigs using proteomics approach. However, in pigs, the information on CAPZB is very limited. In this study, we cloned and characterized the porcine actin capping protein beta (CAPZB) gene. In addition, we present two novel porcine CAPZB splice variants CAPZB1 and CAPZB2. CAPZB1 was expressed in all twenty tissues. However, CAPZB2 was predominantly expressed in the skeletal muscle and heart. In addition, the two isoforms had different expression profiles during the skeletal muscle development and between breeds. Moreover, the SNP T394G was identified in the coding region of the CAPZB gene, which was significantly associated with the carcass traits including the LFW, CFW, SFT and LEA. Data presented in our study suggests that the CAPZB gene may be a candidate gene of meat production trait and provides useful information for further studies on its roles in porcine skeletal muscle.

Subunits of the Drosophila actin-capping protein heterodimer regulate each other at multiple levels

The actin-Capping Protein heterodimer, composed of the α and β subunits, is a master F-actin regulator. In addition to its role in many cellular processes, Capping Protein acts as a main tumor suppressor module in Drosophila and in humans, in part, by restricting the activity of Yorkie/YAP/TAZ oncogenes. We aimed in this report to understand how both subunits regulate each other in vivo. We show that the levels and capping activities of both subunits must be tightly regulated to control F-actin levels and consequently growth of the Drosophila wing. Overexpressing capping protein α and β decreases both F-actin levels and tissue growth, while expressing forms of Capping Protein that have dominant negative effects on F-actin promote tissue growth. Both subunits regulate each other's protein levels. In addition, overexpressing one of the subunit in tissues knocked-down for the other increases the mRNA and protein levels of the subunit knocked-down and compensates for its loss. We propose that the ability of the α and β subunits to control each other's levels assures that a pool of functional heterodimer is produced in sufficient quantities to restrict the development of tumor but not in excess to sustain normal tissue growth.

Proteomic analysis on the alteration of protein expression in the early-stage placental villous tissue of electromagnetic fields associated with cell phone exposure

BACKGROUND

To explore the possible adverse effects and search for cell phone electromagnetic field (EMF)-responsive proteins in human early reproduction, a proteomics approach was employed to investigate the changes in protein expression profile induced by cell phone EMF in human chorionic tissues of early pregnancy in vivo.

METHODS

Volunteer women about 50 days pregnant were exposed to EMF at the average absorption rate of 1.6 to 8.8 W/kg for 1 hour with the irradiation device placed 10 cm away from the umbilicus at the midline of the abdomen. The changes in protein profile were examined using 2-dimensional electrophoresis (2-DE).

RESULTS

Up to 15 spots have yielded significant change at least 2- to 2.5-folds up or down compared to sham-exposed group. Twelve proteins were identified- procollagen-proline, eukaryotic translation elongation factor 1 delta, chain D crystal structure of human vitamin D-binding protein, thioredoxin-like 3, capping protein, isocitrate dehydrogenase 3 alpha, calumenin, Catechol-O-methyltransferase protein, proteinase inhibitor 6 (PI-6; SerpinB6) protein, 3,2-trans-enoyl-CoA isomerase protein, chain B human erythrocyte 2,3-bisphosphoglycerate mutase, and nucleoprotein.

CONCLUSION

Cell phone EMF might alter the protein profile of chorionic tissue of early pregnancy, during the most sensitive stage of the embryos. The exposure to EMF may cause adverse effects on cell proliferation and development of nervous system in early embryos. Furthermore, 2-DE coupled with mass spectrometry is a promising approach to elucidate the effects and search for new biomarkers for environmental toxic effects.

Prognostic value of CAPZA1 overexpression in gastric cancer

F-actin capping protein α1 subunit (CAPZA1) was previously identified in a proteomic analysis of human gastric cancer clinical specimens and selected for further study. The association between CAPZA1 overexpression, detected by immunohistochemistry, and clinicopathological features including survival were evaluated. In vitro gain-of-function and loss-of-function approaches were utilized to assess the function of CPAZA1 in malignancy. Univariate analysis revealed that poorly differentiated disease, according to the World Health Organization (WHO) classification, advanced T stage, positive lymph nodes, high TNM stage, D2 lymph node dissection, adjuvant chemotherapy and CAPZA1 underexpression were significantly associated with cancer-related death (p<0.05); however, only high TNM stage remained significantly associated by multivariate analysis (p<0.01). CAPZA1 overexpression was associated with well differentiated histology, smaller tumor size, lower T stage, absence of lymph node metastasis, lower TNM stage, lower recurrence rate and longer survival time, compared to CAPZA1 underexpression. In vitro, forced expression of CAPZA1 caused a significant decrease in gastric cancer cell migration and invasion, whereas CAPZA1 depletion had the opposite effect. The present study suggests that CAPZA1 could be a marker of good prognosis in gastric cancer and shows that CAPZA1 is associated with decreased cancer cell migration and invasion.

CapZ and actin capping dynamics increase in myocytes after a bout of exercise and abates in hours after stimulation ends

The time course of the response and recovery after acute activity seen in exercise is not well understood. The goal of this work is to address how proteins of the thin filament (actin and its capping protein CapZ) are changed by 1 h of mechanical stimulation and return to baseline over time. Neonatal rat ventricular myocytes in culture were subjected to cyclic 10% strain at 1 Hz for 1 h to mimic increased mechanical loading during exercise. CapZ and actin dynamics were analyzed by fluorescence recovery after photobleaching (FRAP) using CapZβ1-GFP, actin-GFP, or actin-RFP. After cyclic strain, CapZ dynamics increased above resting controls and abated 2-3 h after cessation of the cyclic strain. Similarly, actin dynamics initially increased and abated 1.5-2 h after the end of stimulation. Neurohormonal hypertrophic stimulation by phenylephrine or norepinephrine treatments also elevated actin dynamics but required a much longer time of treatment (24-48 h) to be detectable. The actin capping mechanism was explored by use of expression of CapZβ1 with a COOH-terminal deletion (CapZβ1ΔC). Increased dynamics of actin seen with CapZβ1ΔC was similar to the response to cyclic strain. Thus it is possible that mechanical stimulation alters the dynamics for CapZ capping of the actin filament through the CapZβ1 COOH terminus, known as the β tentacle, thereby remodeling sarcomeres in cardiac myocytes. This adaptive mechanism, which is probably regulating thin-filament addition, declines a few hours after the end of a bout of exercise.

Identification and characterization of the actin-binding motif of phostensin

Phostensin, a protein phosphatase 1 F-actin cytoskeleton-targeting subunit encoded by KIAA1949, consists of 165 amino acids and caps the pointed ends of actin filaments. Sequence alignment analyses suggest that the C-terminal region of phostensin, spanning residues 129 to 155, contains a consensus actin-binding motif. Here, we have verified the existence of an actin-binding motif in the C-terminal domain of phostensin using colocalization, F-actin co-sedimentation and single filament binding assays. Our data indicate that the N-terminal region of phostensin (1-129) cannot bind to actin filaments and cannot retard the pointed end elongation of gelsolin-actin seeds. Furthermore, the C-terminal region of phostensin (125-165) multiply bind to the sides of actin filaments and lacks the ability to block the pointed end elongation, suggesting that the actin-binding motif is located in the C-terminal region of the phostensin. Further analyses indicate that phostensin binding to the pointed end of actin filament requires N-terminal residues 35 to 51. These results suggest that phostensin might fold into a rigid structure, allowing the N-terminus to sterically hinder the binding of C-terminus to the sides of actin filament, thus rendering phostensin binding to the pointed ends of actin filaments.

Bioinformatics analysis of proteomic profiles during the process of anti-Thy1 nephritis

Anti-Thy1 nephritis is a well-established experimental mesangial proliferative nephritis model. Exploring the molecular mechanisms of pathophysiology in anti-Thy1 nephritis may elucidate the pathogeneses of mesangial proliferation. We examined the roles and acting mechanisms of differentially expressed proteins (DEPs) by bioinformatics analysis of glomeruli proteomic profiles during the course of anti-Thy1 nephritis. In total, 108 DEPs were found by two-dimensional fluorescence difference gel electrophoresis (2D-DIGE), and 40 DEPs were identified by matrix-assisted laser desorption ionization/time of flight and liquid chromatography-MS. DEPs were classified into five clusters (Clusters 1-5), according to their expression trends using Cluster 3.0 software, involved in regulating biological processes such as the stress response, cell proliferation, apoptosis, energy metabolism, transport, and the actin cytoskeleton. The expression patterns of ten DEPs, distributed across five clusters, including AKR1A1, AGAT, ATP6V1B2, HIBADH, MDH1, MPST, NIT2, PRDX6, PSMB7, and TPI1, were validated by Western blotting. Based on Western blotting and immunohistochemistry, we also found that the DEP FHL2, which was primarily expressed in the mesangial region, was down-regulated on days 3 and 5, and up-regulated on day 10. In vitro, we found that FHL2 overexpression induced mesangial cell proliferation by increasing the number of S-phase cells and decreasing G2/M-phase cells, whereas inhibiting FHL2 had the opposite effect. This study explored novel DEPs and their expression patterns during anti-Thy1 nephritis, and elucidated FHL2's effect on mesangial cell proliferation. These results will contribute to our understanding of the pathogenesis of mesangial proliferation.

Identification of novel NPRAP/δ-catenin-interacting proteins and the direct association of NPRAP with dynamin 2

Neural plakophilin-related armadillo protein (NPRAP or δ-catenin) is a neuronal-specific protein that is best known for its interaction with presenilin 1 (PS1). Interestingly, the hemizygous loss of NPRAP is associated with severe mental retardation in cri du chat syndrome (CDCS), and mutations in PS1 cause an aggressive, early-onset form of Alzheimer's disease. Until recently, studies on the function of NPRAP have focused on its ability to modulate dendritic protrusion elaboration through its binding to cell adhesion and scaffolding molecules. However, mounting evidence indicates that NPRAP participates in intracellular signaling and exists in the nucleus, where it modulates gene expression. This apparent bifunctional nature suggests an elaborate neuronal role, but how NPRAP came to participate in such distinct subcellular events remains a mystery. To gain insight into this pathway, we immunoprecipitated NPRAP from human SH SY5Y cells and identified several novel interacting proteins by mass spectrometry. These included neurofilament alpha-internexin, interferon regulatory protein 2 binding factors, and dynamins 1 and 2. We further validated dynamin 2/NPRAP colocalization and direct interaction in vivo, confirming their bona fide partnership. Interestingly, dynamin 2 has established roles in endocytosis and actin assembly, and both of these processes have the potential to interface with the cell adhesion and intracellular signaling processes that involve NPRAP. Our data provide new avenues for approaching NPRAP biology and suggest a broader role for this protein than previously thought.

The interaction of capping protein with the barbed end of the actin filament

The interaction of capping protein (CP) with actin filaments is an essential element of actin assembly and actin-based motility in nearly all eukaryotes. The dendritic nucleation model for Arp2/3-based lamellipodial assembly features capping of barbed ends by CP, and the formation of filopodia is proposed to involve inhibition of capping by formins and other proteins. To understand the molecular basis for how CP binds the barbed end of the actin filament, we have used a combination of computational and experimental approaches, primarily involving molecular docking and site-directed mutagenesis. We arrive at a model that supports all of our biochemical data and agrees very well with a cryo-electron microscopy structure of the capped filament. CP interacts with both actin protomers at the barbed end of the filament, and the amphipathic helix at the C-terminus of the β-subunit binds to the hydrophobic cleft on actin, in a manner similar to that of WH2 domains. These studies provide us with new molecular insight into how CP binds to the actin filament.

Site-specific protein adducts of 4-hydroxy-2(E)-nonenal in human THP-1 monocytic cells: protein carbonylation is diminished by ascorbic acid

The protein targets and sites of modification by 4-hydroxy-2(E)-nonenal (HNE) in human monocytic THP-1 cells after exogenous exposure to HNE were examined using a multipronged proteomic approach involving electrophoretic, immunoblotting, and mass spectrometric methods. Immunoblot analysis using monoclonal anti-HNE antibodies showed several proteins as targets of HNE adduction. Pretreatment of THP-1 cells with ascorbic acid resulted in reduced levels of HNE-protein adducts. Biotinylation of Michael-type HNE adducts using an aldehyde-reactive hydroxylamine-functionalized probe (aldehyde-reactive probe, ARP) and subsequent enrichment facilitated the identification and site-specific assignment of the modifications by LC-MS/MS analysis. Sixteen proteins were unequivocally identified as targets of HNE adduction, and eighteen sites of HNE modification at Cys and His residues were assigned. HNE exposure of THP-1 cells resulted in the modification of proteins involved in cytoskeleton organization and regulation, proteins associated with stress responses, and enzymes of the glycolytic and other metabolic pathways. This study yielded the first evidence of site-specific adduction of HNE to Cys-295 in tubulin alpha-1B chain, Cys-351 and Cys-499 in alpha-actinin-4, Cys-328 in vimentin, Cys-369 in D-3-phosphoglycerate dehydrogenase, and His-246 in aldolase A.

Muscle giants: molecular scaffolds in sarcomerogenesis

Myofibrillogenesis in striated muscles is a highly complex process that depends on the coordinated assembly and integration of a large number of contractile, cytoskeletal, and signaling proteins into regular arrays, the sarcomeres. It is also associated with the stereotypical assembly of the sarcoplasmic reticulum and the transverse tubules around each sarcomere. Three giant, muscle-specific proteins, titin (3-4 MDa), nebulin (600-800 kDa), and obscurin (approximately 720-900 kDa), have been proposed to play important roles in the assembly and stabilization of sarcomeres. There is a large amount of data showing that each of these molecules interacts with several to many different protein ligands, regulating their activity and localizing them to particular sites within or surrounding sarcomeres. Consistent with this, mutations in each of these proteins have been linked to skeletal and cardiac myopathies or to muscular dystrophies. The evidence that any of them plays a role as a "molecular template," "molecular blueprint," or "molecular ruler" is less definitive, however. Here we review the structure and function of titin, nebulin, and obscurin, with the literature supporting a role for them as scaffolding molecules and the contradictory evidence regarding their roles as molecular guides in sarcomerogenesis.

Vital role for the Plasmodium actin capping protein (CP) beta-subunit in motility of malaria sporozoites

Successful malaria transmission from the mosquito vector to the mammalian host depends crucially on active sporozoite motility. Sporozoite locomotion and host cell invasion are driven by the parasite's own actin/myosin motor. A unique feature of this motor machinery is the presence of very short subpellicular actin filaments. Therefore, F-actin stabilizing proteins likely play a central role in parasite locomotion. Here, we investigated the role of the Plasmodium berghei actin capping protein (PbCP), an orthologue of the heterodimeric regulator of filament barbed end growth, by reverse genetics. Parasites containing a deletion of the CP beta-subunit developed normally during the pathogenic erythrocytic cycle. However, due to reduced ookinete motility, mutant parasites form fewer oocysts and sporozoites in the Anopheles vector. These sporozoites display a vital deficiency in forward gliding motility and fail to colonize the mosquito salivary glands, resulting in complete attenuation of life cycle progression. Together, our results show that the CP beta-subunit exerts an essential role in the insect vector before malaria transmission to the mammalian host. The vital role is restricted to fast locomotion, as displayed by Plasmodium sporozoites.

Multi-gene gateway clone design for expression of multiple heterologous genes in living cells: eukaryotic clones containing two and three ORF multi-gene cassettes expressed from a single promoter

Two types of eukaryotic operon-type Expression clones were constructed using the Multisite Gateway system employing six types of att signals. These clones harbored a DNA cassette containing two heterologous ORFs (cDNAs) or three heterologous ORFs in tandem downstream of a single promoter. The most promoter-proximal ORF was translated via a Kozak signal and the downstream one or two ORF(s) were translated as directed by internal ribosome entry site(s) (IRES). These clones were observed to produce two or three different proteins at levels that depended on the activities of the translational initiation signals used. With the intention of modulating the expression level of the first ORF, the translational initiation signals including a Kozak sequence and 11 different IRESs were investigated for their efficiency using a single ORF. The translational activity of these signals varied within a 10-fold magnitude. Using these results, expression at pre-described relative levels was achieved from the optional IRES of the respective ORFs in the cassette. Controllable expression at desired levels of two different ORFs directed by optional IRESs on a bicistronic construct, transcribed from a single promoter, was demonstrated.

Multi-gene gateway clone design for expression of multiple heterologous genes in living cells: modular construction of multiple cDNA expression elements using recombinant cloning

Much attention has been focused on manipulating multiple genes in living cells for analyzing protein function. In order to perform high-throughput generation of multi-gene expression clones, gateway cloning technology (which represents a high-throughput DNA transfer from vector to vector) can be anticipated. In the conventional strategy for gateway cloning, the construction of two or more expression elements into tandem elements on a single plasmid requires the recombination of multiple entry clones with a destination vector in a single reaction mixture. Use of increasing numbers of entry clones in a single reaction is inefficient due to the difficulty in successfully recognizing multiple pairs of matched att signals simultaneously. To address this problem, a "Modular Destination" vector has been devised and constructed, whereby cDNA inserts are sequentially introduced, resulting in a tandem structure with multiple inserts. Whereas the standard destination vector contains only Cm(R) and ccdB genes flanked by two attR signals, this destination vector contains, in addition, one or two cDNA expression elements. Here, we show the rapid construction of expression vectors containing three or four tandemly arrayed cDNA expression elements and their expression in mammalian cells.

Structural basis of actin filament capping at the barbed-end: a cryo-electron microscopy study

The intracellular distribution and migration of many protein complexes and organelles is regulated by the dynamics of the actin filament. Many actin filament end-binding proteins play crucial roles in actin dynamics, since polymerization and depolymerization of actin protomers occur only at the filament ends. We present here an EM structure of the complex of the actin filament and hetero-dimeric capping protein (CP) bound to the barbed-end at 23 A resolution, by applying a newly developed methods of image analysis to cryo-electron micrographs. This structure was fitted by the crystal structure of CP and the proposed actin filament structure, allowing us to construct a model that depicts two major binding regions between CP and the barbed-end. This binding scheme accounted for the results of newly performed and previously published mutation experiments, and led us to propose a two-step binding model. This is the first determination of an actin filament end structure.

Multi-gene gateway clone design for expression of multiple heterologous genes in living cells: conditional gene expression at near physiological levels

Using Multisite Gateway five-DNA-fragment constructs vectors that enable expression of two tandemly situated cDNAs on a single plasmid were developed. Heterologous protein production in cells was achieved by modulating respective cDNA expression to pre-determined and different levels. Optimization of cDNA expression at near physiological protein levels was achieved using promoters from four cell cycle-dependent genes. In comparison with conventionally available promoters, EF-1alpha or CMV, the promoters used in this study were able to modulate cDNA expression levels over a magnitude of approximately 10 or 100-fold, respectively. In transiently transfected cells, two different proteins (CPalpha1 and CPbeta2), which form a heterodimer, each labeled with a different-colored fluorescent protein, were successfully synthesized at pre-determined levels from their respective cDNAs. The above vectors were designed to contain an FRT/Flp recombination site for integration onto chromosomes and for establishment of stable clones in HeLa cells by site-specific recombination. In the stable transformant cells produced only about 4% of the protein production levels measured in the transiently transformed cells. The biological significance of these observations is discussed.

Aberrant expression of RAB1A in human tongue cancer

This study was designed to identify specific gene expression changes in tongue squamous cell carcinomas (TSCCs) compared with normal tissues using in-house cDNA microarray that comprised of 2304 full-length cDNAs from a cDNA library prepared from normal oral tissues, primary oral cancers, and oral cancer cell lines. The genes identified by our microarray system were further analysed at the mRNA or protein expression level in a series of clinical samples by real-time quantitative reverse transcriptase–polymerase chain reaction (qRT–PCR) analysis and imuunohositochemistry. The microarray analysis identified a total of 16 genes that were significantly upregulated in common among four TSCC specimens. Consistent with the results of the microarray, increased mRNA levels of selected genes with known molecular functions were found in the four TSCCs. Among genes identified, Rab1a, a member of the Ras oncogene family, was further analysed for its protein expression in 54 TSCCs and 13 premalignant lesions. We found a high prevalence of Rab1A-overexpression not only in TSCCs (98%) but also in premalignant lesions (93%). Thus, our results suggest that rapid characterisation of the target gene(s) for TSCCs can be accomplished using our in-house cDNA microarray analysis combined with the qRT–PCR and immunohistochemistry, and that the Rab1A is a potential biomarker of tongue carcinogenesis.

Extensive analysis of the 7q31 region in human prostate tumors supports TES as the best candidate tumor suppressor gene

Loss of heterozygosity (LOH) on chromosome arm 7q31 is found in many prostate tumors. Such alterations are generally associated with inactivation of tumor suppressor genes. It has been shown previously that the main region of LOH at 7q31 spans the interval between the D7S486 and D7S2460 microsatellite loci, which contains several candidate tumor suppressor genes (TSG) such as TES, CAV2, CAV1, MET, CAPZA2, ST7 and WNT2. We tested 41 human sporadic prostate tumors for 7q31 LOH by using 5 polymorphic markers overlapping the critical region and used a real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay to study the expression of the 7 candidate TSGs located in this genomic region. We found that CAV1, CAV2, MET and TES mRNA expression was lower in prostate tumors than in normal prostate tissues. Our immunohistochemical results and previously published data on the compartmental expression of these messenger RNAs in stromal and epithelial cells suggest that TES is the best candidate tumor suppressor gene at 7q31.

Evidence for high specificity and efficiency of multiple recombination signals in mixed DNA cloning by the Multisite Gateway system

Six types of recombination signal DNA sequences of the Multisite Gateway cloning system were investigated as to their specificity and efficiency in the LR and BP recombination reactions. In the LR reaction to generate an Expression clone by recombination between attL and attR signals which are contained in the Entry clone and the Destination vector, respectively, the cross-reactivity of various attL and attR pairs on six types of respective signal sequences was examined. In the BP reaction to create an Entry clone by transferring the target DNA segment in the Expression clone or the attB-flanked PCR product into a Donor vector, various combinations of attB and attP pairs were tested for their reactivities in recombination. The results obtained indicate a markedly higher specificity and efficiency of cross-reactivity with only the matched att signal pairs, such as attL3-attR3, attB5-attP5, and so on, compared to unmatched signal pairs, such as attL3-attR5, attB5-attP3, and so on, thus verifying a high-throughput production of the positive clones in the Gateway system in which multiple recombination signals exist together in one reaction system. Examples of rapid construction of a three or four DNA-fusion structure in the plasmid are shown.

Effect of Ionizing Irradiation on Human Esophageal Cancer Cell Lines by cDNA Microarray Gene Expression Analysis

To provide new insights into the molecular mechanisms underlying the effect of irradiation on esophageal squamous cell carcinomas (ESCCs), we used a cDNA microarray screening of more than 4,000 genes with known functions to identify genes involved in the early response to ionizing irradiation. Two human ESCC cell lines, one each of well (TE-1) and poorly (TE-2) differentiated phenotypes were screened. Subconfluent cells of each phenotype were treated with single doses of 2.0 Gy or 8.0 Gy irradiations. After a 15 min incubation time-point, the cells were collected and analyzed. Compared with non-irradiated cells, many genes revealed at least 2-fold upregulation or downregulation at both doses in well or poorly differentiated ESCC cells. The common upregulated genes in well and poorly differentiated cell types at both irradiation doses included SCYA5, CYP51, SMARCD2, COX6C, MAPK8, FOS, UBE2M, RPL6, PDGFRL, TRAF2, TNFAIP6, ITGB4, GSTM3, and SP3 and common downregulated genes involved NFIL3, SMARCA2, CAPZA1, MetAP2, CITED2, DAP3, MGAT2, ATRX, CIAO1, and STAT6. Several of these genes were novel and not previously known to be associated with irradiation. Functional annotations of the modulated genes suggested that at the molecular level, irradiation appears to induce a regularizing balance in ESCC cell function. The genes modulated in the early response to irradiation may be useful in our understanding of the molecular basis of radiotherapy and in developing strategies to augment its effect or establish novel less hazardous alternative adjuvant therapies.

Arabidopsis capping protein (AtCP) is a heterodimer that regulates assembly at the barbed ends of actin filaments

The precise regulation of actin filament polymerization and depolymerization is essential for many cellular processes and is choreographed by a multitude of actin-binding proteins (ABPs). In higher plants the number of well characterized ABPs is quite limited, and some evidence points to significant differences in the biochemical properties of apparently conserved proteins. Here we provide the first evidence for the existence and biochemical properties of a heterodimeric capping protein from Arabidopsis thaliana (AtCP). The purified recombinant protein binds to actin filament barbed ends with Kd values of 12-24 nM, as assayed both kinetically and at steady state. AtCP prevents the addition of profilin actin to barbed ends during a seeded elongation reaction and suppresses dilution-mediated depolymerization. It does not, however, sever actin filaments and does not have a preference for the source of actin. During assembly from Mg-ATP-actin monomers, AtCP eliminates the initial lag period for actin polymerization and increases the maximum rate of polymerization. Indeed, the efficiency of actin nucleation of 0.042 pointed ends created per AtCP polypeptide compares favorably with mouse CapZ, which has a maximal nucleation of 0.17 pointed ends per CapZ polypeptide. AtCP activity is not affected by calcium but is sensitive to phosphatidylinositol 4,5-bisphosphate. We propose that AtCP is a major regulator of actin dynamics in plant cells that, together with abundant profilin, is responsible for maintaining a large pool of actin subunits and a surprisingly small population of F-actin.

Linking the T cell surface protein CD2 to the actin-capping protein CAPZ via CMS and CIN85

Recruitment of CD2 to the immunological synapse in response to antigen is dependent on its proline-rich cytoplasmic tail. A peptide from this region (CD2:322-339) isolated CMS (human CD2AP); a related protein, CIN85; and the actin capping protein, CAPZ from a T cell line. In BIAcore analyses, the N-terminal SH3 domains of CMS and CIN85 bound CD2:322-339 with similar dissociation constants (KD = approximately 100 microm). CAPZ bound the C-terminal half of CMS and CIN85. Direct binding between CMS/CIN85 and CAPZ provides a link with the actin cytoskeleton. Overexpression of a fragment from the C-terminal half or the N-terminal SH3 domain of CD2AP in a mouse T cell hybridoma resulted in enhanced interleukin-2 production and reduced T cell receptor down-modulation in response to antigen. These adaptor proteins are important in T cell signaling consistent with a role for CD2 in regulating pathways initiated by CMS/CIN85 and CAPZ.

Crystal structure of CapZ: structural basis for actin filament barbed end capping

Capping protein, a heterodimeric protein composed of alpha and beta subunits, is a key cellular component regulating actin filament assembly and organization. It binds to the barbed ends of the filaments and works as a 'cap' by preventing the addition and loss of actin monomers at the end. Here we describe the crystal structure of the chicken sarcomeric capping protein CapZ at 2.1 A resolution. The structure shows a striking resemblance between the alpha and beta subunits, so that the entire molecule has a pseudo 2-fold rotational symmetry. CapZ has a pair of mobile extensions for actin binding, one of which also provides concomitant binding to another protein for the actin filament targeting. The mobile extensions probably form flexible links to the end of the actin filament with a pseudo 2(1) helical symmetry, enabling the docking of the two in a symmetry mismatch.

Branching out: a molecular fingerprint of endothelial differentiation into tube-like structures generated by Affymetrix oligonucleotide arrays

The process of endothelial differentiation into a network of tube-like structures with patent lumens requires an integrated program of gene expression. To identify genes upregulated in endothelial cells during the process of tube formation, RNA was prepared from several different time points (0, 4, 8, 24, 40, and 48 hours) and from three different experimental models of human endothelial tube formation: in collagen gels and fibrin gels driven by the combination of PMA (80), bFGF (40 ng/ml) and bFGF (40 ng/ml) or in collagen gels driven by the combination of HGF (40 ng/ml) and VEGF (40 ng/ml). Gene expression was evaluated using Affymetrix Gene Chip oligonucleotide arrays. Over 1000 common genes were upregulated greater than twofold over baseline at one or more time points in the three different models. In the present study, we discuss the identified genes that could be assigned to major functional classes: apoptosis, cytoskeleton, proteases, matrix, and matrix turnover, pumps and transporters, membrane lipid turnover, and junctional molecules or adhesion proteins.

Identification of S-glutathionylated cellular proteins during oxidative stress and constitutive metabolism by affinity purification and proteomic analysis

Redox modification of proteins is proposed to play a central role in regulating cellular function. However, high-throughput techniques for the analysis of the redox status of individual proteins in complex mixtures are lacking. The aim was thus to develop a suitable technique to rapidly identify proteins undergoing oxidation of critical thiols by S-glutathionylation. The method is based on the specific reduction of mixed disulfides by glutaredoxin, their reaction with N-ethylmaleimide-biotin, affinity purification of tagged proteins, and identification by proteomic analysis. The method unequivocally identified 43 mostly novel cellular protein substrates for S-glutathionylation. These include protein chaperones, cytoskeletal proteins, cell cycle regulators, and enzymes of intermediate metabolism. Comparisons of the patterns of S-glutathionylated proteins extracted from cells undergoing diamide-induced oxidative stress and during constitutive metabolism reveal both common protein substrates and substrates failing to undergo enhanced S-glutathionylation during oxidative stress. The ability to chemically tag, select, and identify S-glutathionylated proteins, particularly during constitutive metabolism, will greatly enhance efforts to establish posttranslational redox modification of cellular proteins as an important biochemical control mechanism in coordinating cellular function.

Interaction of actin with the capping protein, CapZ from sea bass (Dicentrarchus labrax) white skeletal muscle

We have compared the functional properties of CapZ from fish white skeletal muscle with those of CapZ from chicken muscle. CapZ is a heterodimer, which enhances actin nucleation and inhibits the depolymerization process by binding to the barbed ends of microfilaments. Here, we report the interaction of CapZ not only with F-actin, but also with monomeric actin. The affinity of sea bass CapZ for G-actin estimated by enzyme-linked immunosorbent assay (ELISA) was in the microM range. This association was PIP2 dependent. Binding contacts with the barbed end of actin were delimited by both ELISA and fluorescence approaches. One site (actin sequence 338-348) was located in a helical region of the subdomain 1, region already implicated in the interaction with other actin binding proteins such as gelsolin. Another site implicates the C-terminal region (sequence 360-372) of actin. Finally, the partial competition of antibodies directed against CapZ alpha or beta-subunits towards CapZ interaction with actin filaments suggests both subunits participate in the complex with actin.

Vertebrates have conserved capping protein alpha isoforms with specific expression patterns

Capping protein (CP), a ubiquitous actin binding protein composed of an alpha and a beta subunit, is important for actin assembly and cell motility. Lower organisms have one gene and one isoform of each subunit. Chickens have two very similar alpha-subunit isoforms. To determine if vertebrates in general contain multiple alpha isoforms and if those alpha isoforms have conserved sequences, we isolated and analyzed alpha subunit cDNA's in mice and humans. Both mice and humans also have two alpha isoforms. Phylogenetic analysis of the alpha isoform sequences reveals that vertebrates have two highly conserved subfamilies, alpha1 and alpha2. The alpha1 and alpha2 subfamilies are very similar to each other but can be defined and distinguished from each other by a small number of key amino acid residues. In addition, 3' untranslated cDNA sequences are conserved within the isoform subfamilies. To investigate the function of the alpha isoforms, we examined their expression in mouse cells and tissues. Endothelial cells contain only the alpha2 isoform, and erythrocytes contain almost exclusively the alpha1 isoform. Most tissues have both alpha1 and alpha2 isoforms but the ratio of alpha1:alpha2 varies widely. Together, these findings support the hypothesis that the CP alpha isoforms have conserved, unique and essential roles in vertebrates.

Dissociation of intra- and extracellular domains of desmosomal cadherins and E-cadherin in Hailey-Hailey disease and Darier's disease

In order to clarify the pathomechanism of acantholysis in Hailey-Hailey disease (HHD) and Darier's disease (DD), the distribution of desmosomal and adherens junction-associated proteins was studied in the skin of patients with HHD (n = 4) and DD (n = 3). Domain-specific antibodies were used to determine the cellular localization of the desmosomal transmembrane glycoproteins (desmogleins 1 and 3 and desmocollin), desmosomal plaque proteins (desmoplakin, plakophilin and plakoglobin) and adherens junction-associated proteins (E-cadherin, alpha-catenin, beta-catenin and actin). A significant difference in staining patterns between intra- and extracellular domains of desmosomal cadherins and E-cadherin was demonstrated in acantholytic cells in both HHD and DD, but not in those in pemphigus vulgaris and pemphigus foliaceus samples used as controls. In acantholytic cells in HHD and DD, antibodies against attachment plaque proteins and intracellular epitopes of desmosomal cadherins exhibited diffuse cytoplasmic staining, whereas markedly reduced staining was observed with antibodies against extracellular epitopes of the desmogleins. Similarly, membrane staining of an intracellular epitope of E-cadherin was preserved, while immunoreactivity of an extracellular epitope of E-cadherin was destroyed. While the DD gene has been identified as ATP2A2, the gene for HHD has not been clarified. The dissociation of intra- and extracellular domains of desmosomal cadherin and E-cadherin is characteristic of the acantholytic cells in HHD and DD, and not of pemphigus. This common phenomenon in HHD and DD might be closely related to the pathophysiological mechanisms in both conditions.

In vitro refolding of heterodimeric CapZ expressed in E. coli as inclusion body protein

CapZ is a heterodimeric Ca(2+)-independent actin binding protein which plays an important role in organizing the actin filament lattice of cross-striated muscle cells. It caps the barbed end of actin filaments and promotes nucleation of actin polymerization, thereby regulating actin filament length. Here we report the expression of the two muscle-specific isoforms alpha2 and beta1, from chicken in Escherichia coli as individual subunits using the pQE60 expression vector and the subsequent renaturation of the functional CapZ heterodimer from inclusion bodies. Optimal renaturation conditions were obtained both by simultaneous refolding of urea-solubilized subunits and by rapid dilution into a buffer containing 20% glycerol, 5 mM EGTA, 2 mM DTT, 1 mM PMSF, and 100 mM Tris, pH 7.4. The refolding mixture was incubated for 24 h at 15 degrees C and the protein was concentrated by ultrafiltration. Biochemical characterization of the recombinant heterodimer revealed actin binding activities indistinguishable from those of native CapZ as purified from chicken skeletal muscle. Using the same protocol, we were able to refold the beta1, but not the alpha2 isoform as a single polypeptide, indicating a role for beta1 as a molecular template for the folding of alpha2. The reported recombinant approach leads to high yields of active heterodimer and allows the renaturation and characterization of the beta subunit.

Vertebrate isoforms of actin capping protein beta have distinct functions In vivo

Actin capping protein (CP) binds barbed ends of actin filaments to regulate actin assembly. CP is an alpha/beta heterodimer. Vertebrates have conserved isoforms of each subunit. Muscle cells contain two beta isoforms. beta1 is at the Z-line; beta2 is at the intercalated disc and cell periphery in general. To investigate the functions of the isoforms, we replaced one isoform with another using expression in hearts of transgenic mice. Mice expressing beta2 had a severe phenotype with juvenile lethality. Myofibril architecture was severely disrupted. The beta2 did not localize to the Z-line. Therefore, beta1 has a distinct function that includes interactions at the Z-line. Mice expressing beta1 showed altered morphology of the intercalated disc, without the lethality or myofibril disruption of the beta2-expressing mice. The in vivo function of CP is presumed to involve binding barbed ends of actin filaments. To test this hypothesis, we expressed a beta1 mutant that poorly binds actin. These mice showed both myofibril disruption and intercalated disc remodeling, as predicted. Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform. CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

The role of the dynactin complex in intracellular motility

Dynactin is a multisubunit complex that binds to the minus-end-directed microtubule motor cytoplasmic dynein and may provide a link between the motor and its cargo. Results from genetic studies in Saccharomyces cerevisiae, Neurospora crassa, Aspergillus nidulans, and Drosophila have suggested that cytoplasmic dynein and dynactin function in the same cellular pathways. p150Glued, a vertebrate homologue of the Drosophila gene Glued, is the largest polypeptide in the dynactin complex with multiple protein interactions. Centractin, the most abundant dynactin subunit polypeptide, forms an actin-like filament at the base of the complex. Studies on dynamitin, the 50-kDa dynactin subunit, predict a role for dynactin in mitotic spindle assembly. Other subunits of dynactin have also been cloned and characterized; these studies have provided insight into the role of the complex in essential cellular processes.

CP beta3, a novel isoform of an actin-binding protein, is a component of the cytoskeletal calyx of the mammalian sperm head

In the mammalian sperm head, the nucleus is tightly associated with the calyx, a cell type-specific cytoskeletal structure. Previously, we have identified and characterized some basic proteins such as calicin and cylicins I and II as major calyx components of bovine and human spermatids and spermatozoa. Surprisingly we have now discovered another calyx constituent which by amino acid sequencing and cDNA cloning was recognized as a novel isoform of the widespread beta subunit of the heterodimeric actin-binding "capping protein" (CP). This polypeptide, CP beta3, of sperm calices, is identical with the beta2 subunit present in diverse somatic cell types, except that it shows an amino-terminal extension of 29 amino acids and its mRNA is detected only in testis and, albeit in trace amounts, brain. This CP beta3 mRNA contains the additional sequence, encoded by exon 1 of the gene, which is missing in beta2 mRNAs. Antibodies specific for the beta3 amino-terminal addition have been used to identify the protein by immunoblotting and to localize it to the calyx structure by immunofluorescence microscopy. We conclude that in spermiogenesis the transcription of the gene encoding the beta1, beta2, and beta3 CP subunits is regulated specifically to include exon 1 and to give rise to the testis isoform CP beta3, which is integrated into the calyx structure of the forming sperm head. This surprising finding of an actin-binding protein isoform in an insoluble cytoskeletal structure is discussed in relation to the demonstrated roles of actin and certain actin-binding proteins, such as Limulus alpha-scruin, in spermiogenesis and spermatozoa.

Dynamics of capping protein and actin assembly in vitro: uncapping barbed ends by polyphosphoinositides

Bursts of actin polymerization in vivo involve the transient appearance of free barbed ends. To determine how rapidly barbed ends might appear and how long they might remain free in vivo, we studied the kinetics of capping protein, the major barbed end capper, binding to barbed ends in vitro. First, the off-rate constant for capping protein leaving a barbed end is slow, predicting a half-life for a capped barbed end of approximately 30 min. This half-life implies that cells cannot wait for capping protein to spontaneously dissociate from capped barbed ends in order to create free barbed ends. However, we find that phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-mono-phosphate (PIP) cause rapid and efficient dissociation of capping protein from capped filaments. PIP2 is a strong candidate for a second messenger regulating actin polymerization; therefore, the ability of PIP2 to remove capping protein from barbed ends is a potential mechanism for stimulating actin polymerization in vivo. Second, the on-rate constant for capping protein binding to free barbed ends predicts that actin filaments could grow to the length of filaments observed in vivo during one lifetime. Third, capping protein beta-subunit isoforms did not differ in their actin binding properties, even in tests with different actin isoforms. A major hypothesis for why capping protein beta-subunit isoforms exist is thereby excluded. Fourth, the proposed capping protein regulators, Hsc70 and S100, had no effect on capping protein binding to actin in vitro.

Coordinated regulation of platelet actin filament barbed ends by gelsolin and capping protein

Exposure of cryptic actin filament fast growing ends (barbed ends) initiates actin polymerization in stimulated human and mouse platelets. Gelsolin amplifies platelet actin assembly by severing F-actin and increasing the number of barbed ends. Actin filaments in stimulated platelets from transgenic gelsolin-null mice elongate their actin without severing. F-actin barbed end capping activity persists in human platelet extracts, depleted of gelsolin, and the heterodimeric capping protein (CP) accounts for this residual activity. 35% of the approximately 5 microM CP is associated with the insoluble actin cytoskeleton of the resting platelet. Since resting platelets have an F-actin barbed end concentration of approximately 0.5 microM, sufficient CP is bound to cap these ends. CP is released from OG-permeabilized platelets by treatment with phosphatidylinositol 4,5-bisphosphate or through activation of the thrombin receptor. However, the fraction of CP bound to the actin cytoskeleton of thrombin-stimulated mouse and human platelets increases rapidly to approximately 60% within 30 s. In resting platelets from transgenic mice lacking gelsolin, which have 33% more F-actin than gelsolin-positive cells, there is a corresponding increase in the amount of CP associated with the resting cytoskeleton but no change with stimulation. These findings demonstrate an interaction between the two major F-actin barbed end capping proteins of the platelet: gelsolin-dependent severing produces barbed ends that are capped by CP. Phosphatidylinositol 4,5-bisphosphate release of gelsolin and CP from platelet cytoskeleton provides a mechanism for mediating barbed end exposure. After actin assembly, CP reassociates with the new actin cytoskeleton.

Actin organization, bristle morphology, and viability are affected by actin capping protein mutations in Drosophila

Regulation of actin filament length and orientation is important in many actin-based cellular processes. This regulation is postulated to occur through the action of actin-binding proteins. Many actin-binding proteins that modify actin in vitro have been identified, but in many cases, it is not known if this activity is physiologically relevant. Capping protein (CP) is an actin-binding protein that has been demonstrated to control filament length in vitro by binding to the barbed ends and preventing the addition or loss of actin monomers. To examine the in vivo role of CP, we have performed a molecular and genetic characterization of the beta subunit of capping protein from Drosophila melanogaster. We have identified mutations in the Drosophila beta subunit-these are the first CP mutations in a multicellular organism, and unlike CP mutations in yeast, they are lethal, causing death during the early larval stage. Adult files that are heterozygous for a pair of weak alleles have a defect in bristle morphology that is correlated to disorganized actin bundles in developing bristles. Our data demonstrate that CP has an essential function during development, and further suggest that CP is required to regulate actin assembly during the development of specialized structures that depend on actin for their morphology.

Regulation of actin filament length in erythrocytes and striated muscle

Actin filaments polymerize in vitro to lengths which display an exponential distribution, yet in many highly differentiated cells they can be precisely maintained at uniform lengths in elaborate supramolecular structures. Recent results obtained using two classic model systems, the erythrocyte membrane cytoskeleton and the striated muscle sarcomere, reveal surprising similarities and instructive differences in the molecules and mechanisms responsible for determining and maintaining actin filament lengths in these two systems. Tropomodulin caps the slow-growing, pointed filament ends in muscle and in erythrocytes. CapZ caps the fast-growing, barbed filament ends in striated muscle, whereas a newly discovered barbed end capping protein, adducin, may cap the barbed filament ends in erythrocytes. The mechanisms responsible for specifying the characteristic filament lengths in these systems are more elusive and may include strict control of the relative amounts of actin filament capping proteins and side-binding proteins, molecular templates (e.g. tropomyosin and nebulin) and/or verniers (e.g. tropomyosin).

Cap Z, a calcium insensitive capping protein in resting and activated platelets

Capping of the barbed-ends of actin filaments is an important mechanism for control of the cytoskeleton. In platelets, a valuable model system, it has been thought that gelsolin was the major capping protein. We now report that platelets contain approximately 2 microM Cap Z, a calcium insensitive heterodimeric capping protein; two major and additional minor isoforms of both alpha and beta subunits are present. In lysates from resting platelets 75-80% of the Cap Z sediments with the high speed pellet, but if the platelets are activated with thrombin for 10 s, about 15% of the Cap Z leaves the pellet fraction and is found in the high speed supernatant where it is not bound to actin. This translocation of Cap Z to the supernatant is also observed when resting platelets are lysed into buffer containing 50-100 microM GTP gamma S and 10 mM EGTA. Our results suggest that release of Cap Z from some actin filaments could generate free filament barbed-ends. An increase in free barbed-ends has been reported in platelet lysates prepared shortly after thrombin activation.