Deciphering the nuclear import pathway for the cytoskeletal red cell protein 4.1R

FERM domain-containing proteins are active components of the cell nucleus

The FERM domain is a conserved and widespread protein module that appeared in the common ancestor of amoebae, fungi, and animals, and is therefore now found in a wide variety of species. The primary function of the FERM domain is localizing to the plasma membrane through binding lipids and proteins of the membrane; thus, for a long time, FERM domain-containing proteins (FDCPs) were considered exclusively cytoskeletal. Although their role in the cytoplasm has been extensively studied, the recent discovery of the presence and importance of cytoskeletal proteins in the nucleus suggests that FDCPs might also play an important role in nuclear function. In this review, we collected data on their nuclear localization, transport, and possible functions, which are still scattered throughout the literature, with special regard to the role of the FERM domain in these processes. With this, we would like to draw attention to the exciting, new dimension of the role of FDCPs, their nuclear activity, which could be an interesting novel direction for future research.

Epithelial-specific isoforms of protein 4.1R promote adherens junction assembly in maturing epithelia

Epithelial adherens junctions (AJs) and tight junctions (TJs) undergo disassembly and reassembly during morphogenesis and pathological states. The membrane-cytoskeleton interface plays a crucial role in junctional reorganization. Protein 4.1R (4.1R), expressed as a diverse array of spliceoforms, has been implicated in linking the AJ and TJ complex to the cytoskeleton. However, which specific 4.1 isoform(s) participate and the mechanisms involved in junctional stability or remodeling remain unclear. We now describe a role for epithelial-specific isoforms containing exon 17b and excluding exon 16 4.1R (4.1R+17b) in AJs. 4.1R+17b is exclusively co-localized with the AJs. 4.1R+17b binds to the armadillo repeats 1-2 of β-catenin via its membrane-binding domain. This complex is linked to the actin cytoskeleton via a bispecific interaction with an exon 17b-encoded peptide. Exon 17b peptides also promote fodrin-actin complex formation. Expression of 4.1R+17b forms does not disrupt the junctional cytoskeleton and AJs during the steady-state or calcium-dependent AJ reassembly. Overexpression of 4.1R-17b forms, which displace the endogenous 4.1R+17b forms at the AJs, as well as depletion of the 4.1R+17b forms both decrease junctional actin and attenuate the recruitment of spectrin to the AJs and also reduce E-cadherin during the initial junctional formation of the AJ reassembly process. Expressing 4.1R+17b forms in depleted cells rescues junctional localization of actin, spectrin, and E-cadherin assembly at the AJs. Together, our results identify a critical role for 4.1R+17b forms in AJ assembly and offer additional insights into the spectrin-actin-4.1R-based membrane skeleton as an emerging regulator of epithelial integrity and remodeling.

A 130-kDa protein 4.1B regulates cell adhesion, spreading, and migration of mouse embryo fibroblasts by influencing actin cytoskeleton organization

Protein 4.1B is a member of protein 4.1 family, adaptor proteins at the interface of membranes and the cytoskeleton. It is expressed in most mammalian tissues and is known to be required in formation of nervous and cardiac systems; it is also a tumor suppressor with a role in metastasis. Here, we explore functions of 4.1B using primary mouse embryonic fibroblasts (MEF) derived from wild type and 4.1B knock-out mice. MEF cells express two 4.1B isoforms: 130 and 60-kDa. 130-kDa 4.1B was absent from 4.1B knock-out MEF cells, but 60-kDa 4.1B remained, suggesting incomplete knock-out. Although the 130-kDa isoform was predominantly located at the plasma membrane, the 60-kDa isoform was enriched in nuclei. 130-kDa-deficient 4.1B MEF cells exhibited impaired cell adhesion, spreading, and migration; they also failed to form actin stress fibers. Impaired cell spreading and stress fiber formation were rescued by re-expression of the 130-kDa 4.1B but not the 60-kDa 4.1B. Our findings document novel, isoform-selective roles for 130-kDa 4.1B in adhesion, spreading, and migration of MEF cells by affecting actin organization, giving new insight into 4.1B functions in normal tissues as well as its role in cancer.

Merlin sumoylation is required for its tumor suppressor activity

Merlin, encoded by the Neurofibromatosis 2 (NF2) gene, is a multifunctional tumor suppressor that integrates and regulates extracellular cues and intracellular signaling pathways, both at the plasma membrane and in the nucleus, to control cell proliferation, migration and invasion. Molecular mechanisms regulating merlin's tumor-suppressive activity have not been clearly defined. Here we report that merlin can be sumoylated on Lysine residue (K76) in vitro and in vivo. Sumoylation mediates merlin's intramolecular and intermolecular binding activities and regulates its cytoplasm/nucleus trafficking. Interestingly, sumoylation of merlin is regulated by its phosphorylation via Akt and PAK2 kinases. Mutation of K76 into arginine (R) abolishes its sumoylation, disrupts merlin cortical cytoskeleton residency and attenuates its stability. Using a K76R mutant merlin in a subcutaneous U87MG xenograft model, we demonstrate that merlin sumoylation is required for tumor-suppressive activity. Taken together, our findings indicate that merlin is sumoylated and that this post-translational modification is essential for tumor suppression.

Characterization of cytoskeletal protein 4.1R interaction with NHE1 (Na(+)/H(+) exchanger isoform 1)

NHE1 (Na(+)/H(+) exchanger isoform 1) has been reported to be hyperactive in 4.1R-null erythrocytes [Rivera, De Franceschi, Peters, Gascard, Mohandas and Brugnara (2006) Am. J. Physiol. Cell Physiol. 291, C880-C886], supporting a functional interaction between NHE1 and 4.1R. In the present paper we demonstrate that 4.1R binds directly to the NHE1cd (cytoplasmic domain of NHE1) through the interaction of an EED motif in the 4.1R FERM (4.1/ezrin/radixin/moesin) domain with two clusters of basic amino acids in the NHE1cd, K(519)R and R(556)FNKKYVKK, previously shown to mediate PIP(2) (phosphatidylinositol 4,5-bisphosphate) binding [Aharonovitz, Zaun, Balla, York, Orlowski and Grinstein (2000) J. Cell. Biol. 150, 213-224]. The affinity of this interaction (K(d) = 100-200 nM) is reduced in hypertonic and acidic conditions, demonstrating that this interaction is of an electrostatic nature. The binding affinity is also reduced upon binding of Ca(2+)/CaM (Ca(2+)-saturated calmodulin) to the 4.1R FERM domain. We propose that 4.1R regulates NHE1 activity through a direct protein-protein interaction that can be modulated by intracellular pH and Na(+) and Ca(2+) concentrations.

Structural protein 4.1R is integrally involved in nuclear envelope protein localization, centrosome-nucleus association and transcriptional signaling

The multifunctional structural protein 4.1R is required for assembly and maintenance of functional nuclei but its nuclear roles are unidentified. 4.1R localizes within nuclei, at the nuclear envelope, and in cytoplasm. Here we show that 4.1R, the nuclear envelope protein emerin and the intermediate filament protein lamin A/C co-immunoprecipitate, and that 4.1R-specific depletion in human cells by RNA interference produces nuclear dysmorphology and selective mislocalization of proteins from several nuclear subcompartments. Such 4.1R-deficiency causes emerin to partially redistribute into the cytoplasm, whereas lamin A/C is disorganized at nuclear rims and displaced from nucleoplasmic foci. The nuclear envelope protein MAN1, nuclear pore proteins Tpr and Nup62, and nucleoplasmic proteins NuMA and LAP2α also have aberrant distributions, but lamin B and LAP2β have normal localizations. 4.1R-deficient mouse embryonic fibroblasts show a similar phenotype. We determined the functional effects of 4.1R-deficiency that reflect disruption of the association of 4.1R with emerin and A-type lamin: increased nucleus-centrosome distances, increased β-catenin signaling, and relocalization of β-catenin from the plasma membrane to the nucleus. Furthermore, emerin- and lamin-A/C-null cells have decreased nuclear 4.1R. Our data provide evidence that 4.1R has important functional interactions with emerin and A-type lamin that impact upon nuclear architecture, centrosome-nuclear envelope association and the regulation of β-catenin transcriptional co-activator activity that is dependent on β-catenin nuclear export.

Recognition of nucleoplasmin by its nuclear transport receptor importin α/β: insights into a complete import complex

Nuclear import of the pentameric histone chaperone nucleoplasmin (NP) is mediated by importin α, which recognizes its nuclear localization sequence (NLS), and importin β, which interacts with α and is in charge of the translocation of the NP/α/β complex through the nuclear pore. Herein, we characterize the assembly of a functional transport complex formed by full-length NP with importin α/β. Isothermal titration calorimetry (ITC) was used to analyze the thermodynamics of the interactions of importin α with β, α with NP, and the α/β heterodimer with NP. Our data show that binding of both importin α and α/β to NP is governed by a favorable enthalpic contribution and that NP can accommodate up to five importin molecules per NP pentamer. Phosphomimicking mutations of NP, which render the protein active in histone chaperoning, do not modulate the interaction with importin. Using small-angle X-ray scattering, we model the α/β heterodimer, NP/α, and NP/α/β solution structures, which reveal a glimpse of a complete nuclear import complex with an oligomeric cargo protein. The set of alternative models, equally well fitting the scattering data, yields asymmetric elongated particles that might represent consecutive geometries the complex can adopt when stepping through the nuclear pore.

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.

Merlin/NF2 suppresses tumorigenesis by inhibiting the E3 ubiquitin ligase CRL4(DCAF1) in the nucleus

Current models imply that the FERM domain protein Merlin, encoded by the tumor suppressor NF2, inhibits mitogenic signaling at or near the plasma membrane. Here, we show that the closed, growth-inhibitory form of Merlin accumulates in the nucleus, binds to the E3 ubiquitin ligase CRL4(DCAF1), and suppresses its activity. Depletion of DCAF1 blocks the promitogenic effect of inactivation of Merlin. Conversely, enforced expression of a Merlin-insensitive mutant of DCAF1 counteracts the antimitogenic effect of Merlin. Re-expression of Merlin and silencing of DCAF1 implement a similar, tumor-suppressive program of gene expression. Tumor-derived mutations invariably disrupt Merlin's ability to interact with or inhibit CRL4(DCAF1). Finally, depletion of DCAF1 inhibits the hyperproliferation of Schwannoma cells from NF2 patients and suppresses the oncogenic potential of Merlin-deficient tumor cell lines. We propose that Merlin suppresses tumorigenesis by translocating to the nucleus to inhibit CRL4(DCAF1).

Spectrin repeat proteins in the nucleus

Spectrin repeat sequences are among the more common repeat elements identified in proteins, typically occurring in large structural proteins. Examples of spectrin repeat-containing proteins include dystrophin, alpha-actinin and spectrin itself--all proteins with well-demonstrated roles of establishing and maintaining cell structure. Over the past decade, it has become clear that, although these proteins display a cytoplasmic and plasma membrane distribution, several are also found both at the nuclear envelope, and within the intranuclear space. In this review, we provide an overview of recent work regarding various spectrin repeat-containing structural proteins in the nucleus. As well, we hypothesize about the regulation of their nuclear localization and possible nuclear functions based on domain architecture, known interacting proteins and evolutionary relationships. Given their large size, and their potential for interacting with multiple proteins and with chromatin, spectrin repeat-containing proteins represent strong candidates for important organizational proteins within the nucleus. Supplementary material for this article can be found on the BioEssays website (http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html).

Cell cycle-dependent nucleocytoplasmic shuttling of the neurofibromatosis 2 tumour suppressor merlin

The neurofibromatosis 2 tumour suppressor merlin/schwannomin is structurally related to the ezrin-radixin-moesin family of proteins, which anchor actin cytoskeleton to specific membrane proteins and participate in cell signalling. Merlin inhibits cell growth with a yet unknown mechanism. As most tumour suppressors are linked to cell cycle control, we investigated merlin's behaviour during cell cycle. In glioma and osteosarcoma cells, endogenous merlin was targeted to the nucleus in a cell cycle-specific manner. Merlin accumulated perinuclearly at the G2/M phase, and shifted to the nucleus at early G1. During mitosis, merlin localized to mitotic spindles and at the contractile ring. Nuclear merlin was strongly reduced in confluent cells. Blocking of the CRM1/exportin nuclear export pathway led to accumulation of merlin in the nucleus. Activation of the p21-activated kinase or protein kinase A, which result in phosphorylation of merlin, did not affect its nuclear localization. Merlin regulates the activity of extracellular signal-regulated kinase 2 (ERK2) and nuclear localization of both proteins was induced by cell adhesion. Unlike ERK2, nuclear localization of merlin was not, however, dependent on intact actin cytoskeleton. These results link merlin to events related to cell cycle control and may help to resolve its tumour suppressor function.

Putative tumor suppressor protein 4.1B is differentially expressed in kidney and brain via alternative promoters and 5' alternative splicing

Protein 4.1B has been reported as a tumor suppressor in brain, but not in kidney, despite high expression in both tissues. Here we demonstrate that N-terminal variability in kidney and brain 4.1B isoforms arises through an unusual coupling of RNA processing events in the 5' region of the gene. We describe two transcriptional promoters at far upstream alternative exons 1A and 1B, and show that their respective transcripts splice differentially to exon 2'/2 in a manner that determines mRNA coding capacity. The consequence of this unique processing is that exon 1B transcripts initiate translation at AUG1 (in exon 2') and encode larger 4.1B isoforms with an N-terminal extension; exon 1A transcripts initiate translation at AUG2 (in exon 4) and encode smaller 4.1B isoforms. Tissue-specific differences in promoter utilization may thus explain the abundance of larger 4.1B isoforms in brain but not in kidney. In cell studies, differentiation of PC12 cells was accompanied by translocation of large protein 4.1B isoforms into the nucleus. We propose that first exon specification is coupled to downstream splicing events, generating 4.1B isoforms with diverse roles in kidney and brain physiology, and potentially unique functions in cell proliferation and tumor suppression.

Differential domain evolution and complex RNA processing in a family of paralogous EPB41 (protein 4.1) genes facilitate expression of diverse tissue-specific isoforms

The EPB41 (protein 4.1) genes epitomize the resourcefulness of the mammalian genome to encode a complex proteome from a small number of genes. By utilizing alternative transcriptional promoters and tissue-specific alternative pre-mRNA splicing, EPB41, EPB41L2, EPB41L3, and EPB41L1 encode a diverse array of structural adapter proteins. Comparative genomic and transcript analysis of these 140- to 240-kb genes indicates several unusual features: differential evolution of highly conserved exons encoding known functional domains interspersed with unique exons whose size and sequence variations contribute substantially to intergenic diversity; alternative first exons, most of which map far upstream of the coding regions; and complex tissue-specific alternative pre-mRNA splicing that facilitates synthesis of functionally different complements of 4.1 proteins in various cells. Understanding the splicing regulatory networks that control protein 4.1 expression will be critical to a full appreciation of the many roles of 4.1 proteins in normal cell biology and their proposed roles in human cancer.

Nuclear actin and protein 4.1: essential interactions during nuclear assembly in vitro

Structural protein 4.1, which has crucial interactions within the spectrin-actin lattice of the human red cell membrane skeleton, also is widely distributed at diverse intracellular sites in nucleated cells. We previously showed that 4.1 is essential for assembly of functional nuclei in vitro and that the capacity of 4.1 to bind actin is required. Here we report that 4.1 and actin colocalize in mammalian cell nuclei using fluorescence microscopy and, by higher-resolution detergent-extracted cell whole-mount electron microscopy, are associated on nuclear filaments. We also devised a cell-free assay using Xenopus egg extract containing fluorescent actin to follow actin during nuclear assembly. By directly imaging actin under nonperturbing conditions, the total nuclear actin population is retained and visualized in situ relative to intact chromatin. We detected actin initially when chromatin and nuclear pores began assembling. As nuclear lamina assembled, but preceding DNA synthesis, actin distributed in a reticulated pattern throughout the nucleus. Protein 4.1 epitopes also were detected when actin began to accumulate in nuclei, producing a diffuse coincident pattern. As nuclei matured, actin was detected both coincident with and also independent of 4.1 epitopes. To test whether acquisition of nuclear actin is required for nuclear assembly, the actin inhibitor latrunculin A was added to Xenopus egg extracts during nuclear assembly. Latrunculin A strongly perturbed nuclear assembly and produced distorted nuclear structures containing neither actin nor protein 4.1. Our results suggest that actin as well as 4.1 is necessary for nuclear assembly and that 4.1-actin interactions may be critical.

Identification of a third Protein 4.1 tumor suppressor, Protein 4.1R, in meningioma pathogenesis

Meningiomas are common central nervous system tumors; however, the mechanisms underlying their pathogenesis are largely undefined. In this report, we demonstrate that a third Protein 4.1 family member, Protein 4.1R, functions as a meningioma tumor suppressor. We observed loss of Protein 4.1R expression in two meningioma cell lines (IOMM-Lee, CH157-MN) by Western blotting as well as in 6 of 15 sporadic meningiomas by immunohistochemistry and fluorescence in situ hybridization. In support of a meningioma tumor suppressor function, Protein 4.1R overexpression resulted in reduced IOMM-Lee and CH157-MN cell proliferation. Similar to the Protein 4.1B and merlin tumor suppressors, Protein 4.1R membrane localization increased significantly under conditions of growth arrest in vitro. Lastly, we show that Protein 4.1R interacted with a subset of merlin/Protein 4.1B interactors including CD44 and betaII-spectrin. Collectively, these results suggest that Protein 4.1R functions as an important tumor suppressor in the molecular pathogenesis of meningioma.

Distinct distribution of specific members of protein 4.1 gene family in the mouse nephron

BACKGROUND

Protein 4.1 is an adapter protein that links the actin cytoskeleton to various transmembrane proteins. These 4.1 proteins are encoded by four homologous genes, 4.1R, 4.1G, 4.1N, and 4.1B, which undergo complex alternative splicing. Here we performed a detailed characterization of the expression of specific 4.1 proteins in the mouse nephron.

METHODS

Distribution of renal 4.1 proteins was investigated by staining of paraformaldehyde-fixed mouse kidney sections with antibodies highly specific for each 4.1 protein. Major 4.1 splice forms, amplified from mouse kidney marathon cDNA, were expressed in transfected COS-7 cells in order to assign species of known exon composition to proteins detected in kidney.

RESULTS

A 105 kD 4.1R splice form, initiating at ATG-2 translation initiation site and lacking exon 16, but including exon 17B, was restricted to thick ascending limb of Henle's loop. A 95 kD 4.1N splice form, lacking exons 15 and 17D, was expressed in either descending or ascending thin limb of Henle's loop, distal convoluted tubule, and all regions of the collecting duct system. A major 108 kD 4.1B splice form, initiating at a newly characterized ATG translation initiation site, and lacking exons 15, 17B, and 21, was present only in Bowman's capsule and proximal convoluted tubule (PCT). There was no expression of 4.1G in kidney.

CONCLUSION

Distinct distribution of 4.1 proteins along the nephron suggests their involvement in targeting of selected transmembrane proteins in kidney epithelium and, therefore, in regulation of specific kidney functions.

An alternative domain containing a leucine-rich sequence regulates nuclear cytoplasmic localization of protein 4.1R

In red blood cells, protein 4.1 (4.1R) is an 80-kDa protein that stabilizes the spectrin-actin network and anchors it to the plasma membrane. The picture is more complex in nucleated cells, in which many 4.1R isoforms, varying in size and intracellular location, have been identified. To contribute to the characterization of signals involved in differential intracellular localization of 4.1R, we have analyzed the role the exon 5-encoded sequence plays in 4.1R distribution. We show that exon 5 encodes a leucine-rich sequence that shares key features with nuclear export signals (NESs). This sequence adopts the topology employed for NESs of other proteins and conserves two hydrophobic residues that are shown to be critical for NES function. A 4.1R isoform expressing the leucine-rich sequence binds to the export receptor CRM1 in a RanGTP-dependent fashion, whereas this does not occur in a mutant whose two conserved hydrophobic residues are substituted. These two residues are also essential for 4.1R intracellular distribution, because the 4.1R protein containing the leucine-rich sequence localizes in the cytoplasm, whereas the mutant protein predominantly accumulates in the nucleus. We hypothesize that the leucine-rich sequence in 4.1R controls distribution and concomitantly function of a specific set of 4.1R isoforms.

Two distinct domains of protein 4.1 critical for assembly of functional nuclei in vitro

Protein 4.1R, a multifunctional structural protein, acts as an adaptor in mature red cell membrane skeletons linking spectrin-actin complexes to plasma membrane-associated proteins. In nucleated cells protein 4.1 is not associated exclusively with plasma membrane but is also detected at several important subcellular locations crucial for cell division. To identify 4.1 domains having critical functions in nuclear assembly, 4.1 domain peptides were added to Xenopus egg extract nuclear reconstitution reactions. Morphologically disorganized, replication deficient nuclei assembled when spectrin-actin-binding domain or NuMA-binding C-terminal domain peptides were present. However, control variant spectrin-actin-binding domain peptides incapable of binding actin or mutant C-terminal domain peptides with reduced NuMA binding had no deleterious effects on nuclear reconstitution. To test whether 4.1 is required for proper nuclear assembly, 4.1 isoforms were depleted with spectrin-actin binding or C-terminal domain-specific antibodies. Nuclei assembled in the depleted extracts were deranged. However, nuclear assembly could be rescued by the addition of recombinant 4.1R. Our data establish that protein 4.1 is essential for nuclear assembly and identify two distinct 4.1 domains, initially characterized in cytoskeletal interactions, that have crucial and versatile functions in nuclear assembly.

Decrease in hnRNP A/B expression during erythropoiesis mediates a pre-mRNA splicing switch

A physiologically important alternative pre-mRNA splicing switch, involving activation of protein 4.1R exon 16 (E16) splicing, is required for the establishment of proper mechanical integrity of the erythrocyte membrane during erythropoiesis. Here we identify a conserved exonic splicing silencer element (CE(16)) in E16 that interacts with hnRNP A/B proteins and plays a role in repression of E16 splicing during early erythropoiesis. Experiments with model pre-mRNAs showed that CE(16) can repress splicing of upstream introns, and that mutagenesis or replacement of CE(16) can relieve this inhibition. An affinity selection assay with biotinylated CE(16) RNA demonstrated specific binding of hnRNP A/B proteins. Depletion of hnRNP A/B proteins from nuclear extract significantly increased E16 inclusion, while repletion with recombinant hnRNP A/B restored E16 silencing. Most importantly, differentiating mouse erythroblasts exhibited a stage-specific activation of the E16 splicing switch in concert with a dramatic and specific down-regulation of hnRNP A/B protein expression. These findings demonstrate that natural developmental changes in hnRNP A/B proteins can effect physiologically important switches in pre-mRNA splicing.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

CASK and protein 4.1 support F-actin nucleation on neurexins

Rearrangements of the actin cytoskeleton are involved in a variety of cellular processes from locomotion of cells to morphological alterations of the cell surface. One important question is how local interactions of cells with the extracellular space are translated into alterations of their membrane organization. To address this problem, we studied CASK, a member of the membrane-associated guanylate kinase homologues family of adaptor proteins. CASK has been shown to bind the erythrocyte isoform of protein 4.1, a class of proteins that promote formation of actin/spectrin microfilaments. In neurons, CASK also interacts via its PDZ domain with the cytosolic C termini of neurexins, neuron-specific cell-surface proteins. We now show that CASK binds a brain-enriched isoform of protein 4.1, and nucleates local assembly of actin/spectrin filaments. These interactions can be reconstituted on the cytosolic tail of neurexins. Furthermore, CASK can be recovered with actin filaments prepared from rat brain extracts, and neurexins are recruited together with CASK and protein 4.1 into these actin filaments. Thus, analogous to the PDZ-domain protein p55 and glycophorin C at the erythrocyte membrane, a similar complex comprising CASK and neurexins exists in neurons. Our data suggest that intercellular junctions formed by neurexins, such as junctions initiated by beta-neurexins with neuroligins, are at least partially coupled to the actin cytoskeleton via an interaction with CASK and protein 4.1.

Biophysical characterization of interactions involving importin-alpha during nuclear import

Proteins containing the classical nuclear localization sequences (NLSs) are imported into the nucleus by the importin-alpha/beta heterodimer. Importin-alpha contains the NLS binding site, whereas importin-beta mediates the translocation through the nuclear pore. We characterized the interactions involving importin-alpha during nuclear import using a combination of biophysical techniques (biosensor, crystallography, sedimentation equilibrium, electrophoresis, and circular dichroism). Importin-alpha is shown to exist in a monomeric autoinhibited state (association with NLSs undetectable by biosensor). Association with importin-beta (stoichiometry, 1:1; K(D) = 1.1 x 10(-8) m) increases the affinity for NLSs; the importin-alpha/beta complex binds representative monopartite NLS (simian virus 40 large T-antigen) and bipartite NLS (nucleoplasmin) with affinities (K(D) = 3.5 x 10(-8) m and 4.8 x 10(-8) m, respectively) comparable with those of a truncated importin-alpha lacking the autoinhibitory domain (T-antigen NLS, K(D) = 1.7 x 10(-8) m; nucleoplasmin NLS, K(D) = 1.4 x 10(-8) m). The autoinhibitory domain (as a separate peptide) binds the truncated importin-alpha, and the crystal structure of the complex resembles the structure of full-length importin-alpha. Our results support the model of regulation of nuclear import mediated by the intrasteric autoregulatory sequence of importin-alpha and provide a quantitative description of the binding and regulatory steps during nuclear import.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

Regulation of alternative pre-mRNA splicing during erythroid differentiation

Although the mature enucleated erythrocyte is no longer active in nuclear processes such as pre-mRNA splicing, the function of many of its major structural proteins is dependent on alternative splicing choices made during the earlier stages of erythropoiesis. These splicing decisions fundamentally regulate many aspects of protein structure and function by governing the inclusion or exclusion of exons that encode protein interaction domains, regulatory signals, or translation initiation or termination sites. Alternative splicing events may be partially or entirely erythroid-specific, ie, distinct from the splicing patterns imposed on the same transcripts in nonerythroid cells. Moreover, differentiation stage-specific splicing "switches" may alter the structure and function of erythroid proteins in physiologically important ways as the cell is morphologically and functionally remodeled during normal differentiation. Derangements in the splicing of individual mutated pre-mRNAs can produce synthesis of truncated or unstable proteins that are responsible for numerous erythrocyte disorders. This review will summarize the salient features of regulated alternative splicing in general, review existing information concerning the widespread extent of alternative splicing among erythroid genes, and describe recent studies that are beginning to uncover the mechanisms that regulate an erythroid splicing switch in the protein 4.1R gene.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Characterization of the interaction between protein 4.1R and ZO-2. A possible link between the tight junction and the actin cytoskeleton

Multiple isoforms of the red cell protein 4.1R are expressed in nonerythroid cells, including novel 135-kDa isoforms. Using a yeast two-hybrid system, immunocolocalization, immunoprecipitation, and in vitro binding studies, we found that two 4.1R isoforms of 135 and 150 kDa specifically interact with the protein ZO-2 (zonula occludens-2). 4.1R is colocalized with ZO-2 and occludin at Madin-Darby canine kidney (MDCK) cell tight junctions. Both isoforms of 4.1R coprecipitated with proteins that organize tight junctions such as ZO-2, ZO-1, and occludin. Western blot analysis also revealed the presence of actin and alpha-spectrin in these immunoprecipitates. Association of 4.1R isoforms with these tight junction and cytoskeletal proteins was found to be specific for the tight junction and was not seen in nonconfluent MDCK cells. The amino acid residues that sustain the interaction between 4.1R and ZO-2 reside within the amino acids encoded by exons 19-21 of 4.1R and residues 1054-1118 of ZO-2. Exogenously expressed 4.1R containing the spectrin/actin- and ZO-2-binding domains was recruited to tight junctions in confluent MDCK cells. Taken together, our results suggest that 4.1R might play an important role in organization and function of the tight junction by establishing a link between the tight junction and the actin cytoskeleton.

A constitutive region is responsible for nuclear targeting of 4.1R: modulation by alternative sequences results in differential intracellular localization

Red blood cell protein 4.1, 4.1R, is an extreme variation on the theme of isoform multiplicity. The diverse 4.1R isoforms, mainly generated by alternative pre-mRNA splicing, are localized at different intracellular sites, including the nucleus. To characterize nonerythroid 4.1 proteins lacking the most upstream translation initiation site, analyze their intracellular localization and define specific domains involved in differential intracellular targeting of 4.1R, we cloned 4.1 cDNAs lacking that translation initiation site. Seven different 4.1R cDNAs were isolated. Four of these encoded 4.1R proteins localized predominantly to the nucleus and the other three localized to the cytoplasm. Three of the nuclear 4.1R isoforms did not contain the nuclear localization signal previously identified in the alternative exon 16. A comparative analysis of the exon composition of the naturally occurring 4.1R cDNAs cloned and of appropriate composite cDNA constructs, with the subcellular distribution of their respective products, demonstrated that a region encoded by constitutive exons, which is therefore common to all 4.1R isoforms and has been termed ‘core region’, had the capacity of localizing to the nucleus. This region was able to confer nuclear targeting to a cytosolic reporter. In protein 4.1R isoforms, the nuclear targeting of the core region is modulated by the expression of alternative exons. Thus, exon 5-encoded sequences eclipsed nuclear entry of the core region, resulting in 4.1R isoforms that predominantly distributed to the cytoplasm. Exon 5 was also able to confer cytoplasmic localization to a nuclear reporter. In protein 4.1R isoforms, when exons 5 and 16 were both expressed the nuclear targeting effect of exon 16 was dominant to the inhibitory effect observed by the expression of exon 5, yielding proteins that predominantly localized to the nucleus. Taken together, these results indicate that all 4.1R molecules contain a conserved region that is sufficient to target the protein to the nucleus, but that specific exon-encoded sequences modulate this capacity by acting in a hierarchical order.

Nuclear targeting signal recognition: a key control point in nuclear transport?

Recent progress indicates that there are multiple pathways of nucleocytoplasmic transport which involve specific targeting sequences, such as nuclear localization sequences (NLSs), and cytosolic receptor molecules of the importin/karyopherin superfamily which recognise and dock the NLS-containing proteins at the nuclear pore. This first step of nuclear import/export is of central importance, with the affinity of the importin-targeting sequence interaction a critical parameter in determining transport efficiency. Different importins possess distinct NLS-binding specificities, which allows the system to be modulated through differential expression of the importins themselves, as well as through competition between different importins for the same protein, and between different proteins for the same importin. The targeting sequence-importin interaction can also be influenced directly by phosphorylation increasing the affinity of the interaction with importins or by targeting sequence masking through phosphorylation or specific protein binding. Targeting sequence recognition thus appears to represent a key control point in the regulation of nuclear transport. BioEssays 22:532-544, 2000.

New insights into functions of erythroid proteins in nonerythroid cells

This article presents new insights into potential roles that three erythrocyte cytoskeletal proteins, protein 4.1, ankyrin, and spectrin, may play in nonerythroid nucleated cells. Each of these proteins is encoded by several closely related genes characterized by complex alternative splicing of their pre-mRNA, thus resulting in the cellular expression of a broad repertoire of isoforms that can adopt tissue- and cell-specific distribution. This could account for the presence of skeletal networks in intracellular organelles such as lysosomes, the Golgi apparatus, or the nucleus. In addition to providing structural support to cell membranes, these cytoskeletal proteins regulate the functions of various transmembrane proteins they interact with, in particular ion channels, as well as the activity of membrane-bound enzymes. Thus, they appear to be key players in major unsuspected cell functions such as protein sorting, dynamics of nuclear architecture during mitosis, or regulation of signal transduction pathways.

Molecular and functional characterization of protein 4.1B, a novel member of the protein 4.1 family with high level, focal expression in brain

Brain-enriched isoforms of skeletal proteins in the spectrin and ankyrin gene families have been described. Here we characterize protein 4.1B, a novel homolog of erythrocyte protein 4.1R that is encoded by a distinct gene. In situ hybridization revealed high level, focal expression of 4.1B mRNA in select neuronal populations within the mouse brain, including Purkinje cells of the cerebellum, pyramidal cells in hippocampal regions CA1-3, thalamic nuclei, and olfactory bulb. Expression was also detected in adrenal gland, kidney, testis, and heart. 4.1B protein exhibits high homology to the membrane binding, spectrin-actin binding, and C-terminal domains of 4.1R, including motifs for interaction with NuMA and FKBP13. cDNA characterization and Western blot analysis revealed multiple spliceoforms of protein 4.1B, with functionally relevant heterogeneity in the spectrin-actin and NuMA binding domains. Regulated alternative splicing events led to expression of unique 4. 1B isoforms in brain and muscle; only the latter possessed a functional spectrin-actin binding domain. By immunofluorescence, 4. 1B was localized specifically at the plasma membrane in regions of cell-cell contact. Together these results indicate that 4.1B transcription is selectively regulated among neuronal populations and that alternative splicing regulates expression of 4.1B isoforms possessing critical functional domains typical of other protein 4.1 family members.

Alternative splicing of protein 4.1R exon 16: ordered excision of flanking introns ensures proper splice site choice

Alternative splicing plays a major role in regulating tissue-specific expression of cytoskeletal protein 4.1R isoforms. In particular, expression of the protein's functionally critical spectrin-actin binding domain, essential for maintenance of red cell membrane mechanical properties, is governed by a developmentally regulated splicing switch involving alternative exon 16. Using a model 3-exon 4.1R pre–messenger RNA (pre-mRNA), we explored the sequence requirements for excision of the introns flanking exon 16. These studies revealed that splicing of this alternative exon occurs preferentially in an ordered fashion. The first step is excision of the downstream intron to join exons 16 and 17, followed by excision of the upstream intron. Constructs designed to test the converse pathway were spliced less efficiently and with less fidelity, in part due to activation of a cryptic 5′ splice site in exon 16. This downstream-first model for ordered splicing is consistent with the hypothesis that regulated alternative splicing requires cooperation between multiple exonic and/or intronic regulatory elements whose spatial organization is critical for recruitment of appropriate splicing factors. Our results predict that exon 16 splicing is regulated at the first step—excision of the downstream intron—and that cells unable to catalyze this step will exhibit exon 16 skipping. In cells that include exon 16, adherence to an ordered pathway is important for efficient and accurate production of mature 4.1R mRNA encoding an intact spectrin-actin binding domain.

Protein 4.1N binding to nuclear mitotic apparatus protein in PC12 cells mediates the antiproliferative actions of nerve growth factor

Protein 4.1N is a neuronal selective isoform of the erythrocyte membrane cytoskeleton protein 4.1R. In the present study, we demonstrate an interaction between 4.1N and nuclear mitotic apparatus protein (NuMA), a nuclear protein required for mitosis. The binding involves the C-terminal domain of 4.1N. In PC12 cells treatment with nerve growth factor (NGF) elicits translocation of 4. 1N to the nucleus and promotes its association with NuMA. Specific targeting of 4.1N to the nucleus arrests PC12 cells at the G1 phase and produces an aberrant nuclear morphology. Inhibition of 4.1N nuclear translocation prevents the NGF-mediated arrest of cell division, which can be reversed by overexpression of 4.1N. Thus, nuclear 4.1N appears to mediate the antiproliferative actions of NGF by antagonizing the role of NuMA in mitosis.

Protein 4.1N binding to nuclear mitotic apparatus protein in PC12 cells mediates the antiproliferative actions of nerve growth factor

Protein 4.1N is a neuronal selective isoform of the erythrocyte membrane cytoskeleton protein 4.1R. In the present study, we demonstrate an interaction between 4.1N and nuclear mitotic apparatus protein (NuMA), a nuclear protein required for mitosis. The binding involves the C-terminal domain of 4.1N. In PC12 cells treatment with nerve growth factor (NGF) elicits translocation of 4. 1N to the nucleus and promotes its association with NuMA. Specific targeting of 4.1N to the nucleus arrests PC12 cells at the G1 phase and produces an aberrant nuclear morphology. Inhibition of 4.1N nuclear translocation prevents the NGF-mediated arrest of cell division, which can be reversed by overexpression of 4.1N. Thus, nuclear 4.1N appears to mediate the antiproliferative actions of NGF by antagonizing the role of NuMA in mitosis.

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.