Molecular characterization of human zyxin

Actin-based endosome and phagosome rocketing in macrophages: activation by the secretagogue antagonists lanthanum and zinc

Although motile endocytic vesicles form actin-rich rocket tails [Merrifield et al., 1999: Nature Cell Biol 1:72-74], the mechanism of intracellular organelle locomotion remains poorly understood. We now demonstrate that bone marrow macrophages treated with lanthanum and zinc ions, well-known secretagogue antagonists, reliably exhibit vesicle motility. This treatment results in accentuated membrane ruffling and the formation of phagosomes and early endosomes that move rapidly through the cytoplasm by assembling actin filament rocket tails. Protein-specific immunolocalization demonstrated the presence of Arp2/3 complex in the polymerization zone and throughout the actin-rich tail, whereas N-WASP was most abundant in the polymerization zone. Although Arp2/3 and N-WASP play essential roles in nucleating filament assembly, other processes (i.e., elongation and filament cross-linking) are required to produce forces needed for motility. Efficient elongation was found to require zyxin, VASP, and profilin, proteins that interact by means of their ABM-1 and ABM-2 proline-rich motifs. The functional significance of these motifs was demonstrated by inhibition of vesicle motility by the motif-specific ABM-1 and ABM-2 analogues. Furthermore, lanthanum/zinc treatment also facilitated the early onset of actin-based vaccinia motility, a process that also utilizes Arp2/3 and N-WASP for nucleation and the zyxin-VASP-profilin complex for efficient elongation. Although earlier studies using cell extracts clouded the role of oligoproline sequences in activating the polymerization zone, our studies emphasize the importance of evaluating motility in living cells.

Targeted disruption of the murine zyxin gene

Zyxin is an evolutionarily conserved protein that is concentrated at sites of cell adhesion, where it associates with members of the Enabled (Ena)/vasodilator-stimulated phosphoprotein (VASP) family of cytoskeletal regulators and is postulated to play a role in cytoskeletal dynamics and signaling. Zyxin transcripts are detected throughout murine embryonic development, and the protein is widely expressed in adults. Here we used a reverse genetic approach to examine the consequences of loss of zyxin function in the mouse. Mice that lack zyxin function are viable and fertile and display no obvious histological abnormalities in any of the organs examined. Because zyxin contributes to the localization of Ena/VASP family members at certain subcellular locations, we carefully examined the zyxin(-/-) mice for evidence of defects that have been observed when Ena/VASP proteins are compromised in the mouse. Specifically, we evaluated blood platelet function, nervous system development, and skin architecture but did not detect any defects in these systems. Zyxin is the founding member of a family of proteins that also includes the lipoma preferred partner (LPP) and thyroid receptor-interacting protein 6 (TRIP6). These zyxin family members display patterns of expression that significantly overlap that of zyxin. Western blot analysis indicates that there is no detectable upregulation of either LPP or TRIP6 expression in tissues derived from zyxin-null mice. Because zyxin family members may have overlapping functions, a comprehensive understanding of the role of these proteins in the mouse will require the generation of compound mutations in which multiple zyxin family members are simultaneously compromised.

Rabbit epididymal secretory proteins. I. Characterization and hormonal regulation

Analyses of samples of luminal fluid from the rete testis, distal efferent ducts, and epididymal regions 2-5 and 8 revealed that 91% of the fluid leaving the testis is reabsorbed by the efferent ducts, 79% of the remainder is reabsorbed proximal to epididymal regions 4 and 5, and there is a net secretion of fluid into the duct caudally. There is a net reabsorption by the efferent ducts of 73% of the protein leaving the testis and then a net secretion along the epididymis. SDS-PAGE of the luminal fluids indicated that four new protein bands that were not present in blood appeared in the efferent ducts, 5 in epididymal regions 1-5, 6 in regions 6 and 7, and one in region 8. Two bands in samples from the efferent ducts were absent caudally, and one band present in region 7 was absent in region 8. The rates of incorporation of (35)S-methionine into minced duct in vitro varied among regions when expressed per milligram of wet weight of tissue (region 2-5 > region 7 > region 6 > region 1 > region 8 > ductuli efferentes), and orchidectomy had little effect on the rates. Incorporation into four proteins that were secreted in vitro (M(r) 38 000, 20 000, 15 000, and 13 000) was reduced or abolished by orchidectomy and restored by testosterone therapy. The secretion of three proteins (M(r) 52 000, 23 000, and 22 000) was reduced or abolished by orchidectomy and not restored by testosterone therapy. SDS-PAGE of detergent extracts of sperm indicated that five proteins were lost and nine were gained during epididymal transit. Seven of the proteins gained were about the same molecular weight as proteins secreted by the epididymis (M(r) 94 000, 52 000, 38 000, 36 000, 22 000, 20 000, and 13 000) and were analyzed using N-terminal amino acid microsequencing.

Members of the Zyxin family of LIM proteins interact with members of the p130Cas family of signal transducers

Integrin binding to extracellular matrix proteins induces formation of signaling complexes at focal adhesions. Zyxin co-localizes with integrins at sites of cell-substratum adhesion and is postulated to serve as a docking site for the assembly of multimeric protein complexes involved in regulating cell motility. Recently, we identified a new member of the zyxin family called TRIP6. TRIP6 is localized at focal adhesions and overexpression of TRIP6 slows cell migration. In an effort to define the molecular mechanism by which TRIP6 affects cell migration, the yeast two-hybrid assay was employed to identify proteins that directly bind to TRIP6. This assay revealed that both TRIP6 and zyxin interact with CasL/HEF1, a member of the Cas family. This association is mediated by the LIM region of the zyxin family members and the SH2 domain-binding region of CasL/HEF1. Furthermore, the association between p130(Cas) and the two zyxin family members was demonstrated to occur in vivo by co-immunoprecipitation. Zyxin and Cas family members may cooperate to regulate cell motility.

Cytokine-induced down-regulation of zfm1/splicing factor-1 promotes smooth muscle cell proliferation

One hallmark of inflammation is the proliferation of bystander cells such as vascular smooth muscle cells (SMC), a process governed by growth factors and cytokines. Whereas cytokine induction of gene products promoting inflammation and proliferation is well characterized, little is known about the concomitant down-regulation of potentially counter-regulatory gene products in these cells. By employing the suppression subtractive hybridization-PCR technique, RNA isolated from rat aortic SMC treated with the cytokines interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) was subtracted from RNA of control cells. Eleven genes were identified, the expression of which fell by 44-77%. One, the transcriptional repressor splicing factor-1 or zfm1, was characterized further. Antisense oligonucleotide suppression of zfm1 protein synthesis mimicked the stimulatory effects of IL-1 beta and TNF alpha on SMC proliferation and expression of the chemokine MCP-1 and the vascular cell adhesion molecule-1. Moreover, in an in vivo mouse model of atherosclerosis, zfm1 abundance was decreased in proliferating arterial SMC. These findings suggest a role for zfm1 in controlling both proliferation and expression of pro-inflammatory gene products in SMC. Therefore, cytokine-induced down-regulation of zfm1 expression may contribute to the pathogenesis of hyperproliferative inflammatory diseases.

Role of zyxin in differential cell spreading and proliferation of melanoma cells and melanocytes

Cell spreading, proliferation, and survival are modulated by focal adhesions linking extracellular matrix proteins, integrins, and the cytoskeleton. Zyxin is a focal-adhesion-associated phosphoprotein with one domain involved in the control of actin assembly and three protein-protein adapter domains implicated in the regulation of cell growth and differentiation. We characterized zyxin expression in normal human melanocytes and six melanoma cell lines in relation to cell spreading, growth, and differentiation using Western immunoblotting techniques, image analysis, flow cytometry, and confocal microscopy. We found that zyxin, focal adhesion kinase, and paxillin were significantly upregulated in melanoma cells compared to melanocytes. Zyxin expression directly related to cell spreading and proliferation and inversely related to differentiation, whereas focal adhesion kinase correlated only to cell spreading and paxillin did not significantly correlate with any of the parameters. Treatment of melanoma cells with 12-O-tetradecanoylphorbol-13-acetate downregulated zyxin expression, inhibited cell spreading and proliferation, and promoted differentiation. In contrast, 12-O-tetradecanoylphorbol-13-acetate, a mitogen for melanocytes, induced upregulation of zyxin expression in melanocytes. These findings are consistent with a role of zyxin in modulation of cell spreading, proliferation, and differentiation. Therapies directed at the downregulation of this focal adhesion phosphoprotein in melanoma cells implicate a new approach for controlling melanoma cell growth.

Interaction of zyxin, a focal adhesion protein, with the e6 protein from human papillomavirus type 6 results in its nuclear translocation

Zyxin, a focal adhesion molecule, interacts specifically with the E6 protein from human papillomavirus (HPV) type 6 in a yeast two-hybrid screen of a cDNA library prepared from human keratinocytes. Zyxin does not interact significantly with E6 proteins from HPV types 11, 16, or 18. The interaction was confirmed by in vitro and in vivo analyses and it requires the LIM domains (Lin-11, Isl-1, and Mec-3 [G. Freyd, S. K. Kim, and H. R. Horvitz, Nature 344:876-879, 1990]) found at the carboxyl terminus of zyxin. Cotransfection of E6 from HPV ((6)E6) and zyxin results in the accumulation of zyxin in the nucleus where it can function as a transcriptional activator. (6)E6 can also mobilize endogenous zyxin to the nucleus.

Zyxin is not colocalized with vasodilator-stimulated phosphoprotein (VASP) at lamellipodial tips and exhibits different dynamics to vinculin, paxillin, and VASP in focal adhesions

Actin polymerization is accompanied by the formation of protein complexes that link extracellular signals to sites of actin assembly such as membrane ruffles and focal adhesions. One candidate recently implicated in these processes is the LIM domain protein zyxin, which can bind both Ena/vasodilator-stimulated phosphoprotein (VASP) proteins and the actin filament cross-linking protein alpha-actinin. To characterize the localization and dynamics of zyxin in detail, we generated both monoclonal antibodies and a green fluorescent protein (GFP)-fusion construct. The antibodies colocalized with ectopically expressed GFP-VASP at focal adhesions and along stress fibers, but failed to label lamellipodial and filopodial tips, which also recruit Ena/VASP proteins. Likewise, neither microinjected, fluorescently labeled zyxin antibodies nor ectopically expressed GFP-zyxin were recruited to these latter sites in live cells, whereas both probes incorporated into focal adhesions and stress fibers. Comparing the dynamics of zyxin with that of the focal adhesion protein vinculin revealed that both proteins incorporated simultaneously into newly formed adhesions. However, during spontaneous or induced focal adhesion disassembly, zyxin delocalization preceded that of either vinculin or paxillin. Together, these data identify zyxin as an early target for signals leading to adhesion disassembly, but exclude its role in recruiting Ena/VASP proteins to the tips of lamellipodia and filopodia.

Targeting of zyxin to sites of actin membrane interaction and to the nucleus

The localization of proteins to particular intracellular compartments often regulates their functions. Zyxin is a LIM protein found prominently at sites of cell adhesion, faintly in leading lamellipodia, and transiently in cell nuclei. Here we have performed a domain analysis to identify regions in zyxin that are responsible for targeting it to different subcellular locations. The N-terminal proline-rich region of zyxin, which harbors binding sites for alpha-actinin and members of the Ena/VASP family, concentrates in lamellipodial extensions and weakly in focal adhesions. The LIM region of zyxin displays robust targeting to focal adhesions. When overexpressed in cells, the LIM region of zyxin causes displacement of endogenous zyxin from focal adhesions. Upon mislocalization of full-length zyxin, at least one member of the Ena/VASP family is also displaced, and the organization of the actin cytoskeleton is perturbed. Zyxin also has the capacity to shuttle between the nucleus and focal adhesion sites. When nuclear export is inhibited, zyxin accumulates in cell nuclei. The nuclear accumulation of zyxin occurs asynchronously with approximately half of the cells exhibiting nuclear localization of zyxin within 2.3 h of initiating leptomycin B treatment. Our results provide insight into the functions of different zyxin domains.

Analysis of the alpha-actinin/zyxin interaction

The yeast two-hybrid system was used to search for interaction partners of human zyxin. Screening of two different cDNA libraries, one prepared from human placenta, the other from human heart, yielded several positive clones that occurred in both searches, including clones coding for cyclophilin, nebulette, and alpha-actinin. The zyxin/alpha-actinin interaction was analyzed in detail. By site-directed mutagenesis, a linear motif of 6 amino acids (Phe-Gly-Pro-Val-Val-Ala) present at the N terminus of zyxin was found to play a critical role. Replacement of a single amino acid within this motif abolished binding to alpha-actinin in blot overlays as well as in living cells. On the other hand, the interaction site in alpha-actinin was mapped to a conformational determinant present in the center of the protein as demonstrated by a fragment deletion analysis. This binding site involved a tandem array of two complete spectrin-like domains. Only fragments that were able to dimerize in yeast also bound to zyxin, suggesting that dimerization of alpha-actinin is essential for zyxin binding.

ActA and human zyxin harbour Arp2/3-independent actin-polymerization activity

The actin cytoskeleton is a dynamic network that is composed of a variety of F-actin structures. To understand how these structures are produced, we tested the capacity of proteins to direct actin polymerization in a bead assay in vitro and in a mitochondrial-targeting assay in cells. We found that human zyxin and the related protein ActA of Listeria monocytogenes can generate new actin structures in a vasodilator-stimulated phosphoprotein-dependent (VASP) manner, but independently of the Arp2/3 complex. These results are consistent with the concept that there are multiple actin-polymerization machines in cells. With these simple tests it is possible to probe the specific function of proteins or identify novel molecules that act upon cellular actin polymerization.

Translocation of a human focal adhesion LIM-only protein, FHL2, during myofibrillogenesis and identification of LIM2 as the principal determinants of FHL2 focal adhesion localization

LIM domain proteins are found to be important regulators in cell growth, cell fate determination, cell differentiation, and remodeling of the cell cytoskeleton. Human Four-and-a-half LIM-only protein 2 (FHL2) is expressed predominantly in human heart and is only slightly expressed in skeletal muscle. Since FHL2 is an abundant protein in human heart, it may play an important role in the regulation of cell differentiation and myofibrillogenesis of heart at defined subcellular compartment. Therefore, we hypothesized that FHL2 act as a multi-functional protein by the specific arrangement of the LIM domains of FHL2 and that one of the LIM domains of FHL2 can function as an anchor and localizes it into a specific subcellular compartment in a cell type specific manner to regulate myofibrillogenesis. From our results, we observed that FHL2 is localized at the focal adhesions of the C2C12, H9C2 myoblast as well as a nonmyogenic cell line, HepG2 cells. Colocalization of vinculin-CFP and FHL2-GFP at focal adhesions was also observed in cell lines. Site-directed mutagenesis, in turn, suggested that the second LIM domain-LIM2 is essential for its specific localization to focal adhesions. Moreover, FHL2 was observed along with F-actin and focal adhesion of C2C12 and H9C2 myotubes. Finally, we believe that FHL2 moves from focal adhesions and then stays at the Z-discs of terminally differentiated heart muscle.

The orthologous human and murine semaphorin 6A-1 proteins (SEMA6A-1/Sema6A-1) bind to the enabled/vasodilator-stimulated phosphoprotein-like protein (EVL) via a novel carboxyl-terminal zyxin-like domain

Neuronal development and apoptosis critically depend on the transformation of extracellular signals to intracellular actions resulting in cytoskeletal rearrangements. Ena/VASP (enabled/vasodilator-stimulated phosphoprotein) proteins play an important role in actin and filament dynamics, whereas members of the semaphorin protein family are guidance signals in embryo- and organogenesis. Here, we report the identification of two novel transmembranous human and murine semaphorins, (HSA)SEMA6A-1 and (MMU)Sema6A-1. These semaphorin 6 variants directly link the Ena/VASP and the semaphorin protein family, since SEMA6A-1/Sema6A-1 is capable of a selective binding to the protein EVL (Ena/VASP-like protein). EVL is the third member of the Ena/VASP family of proteins that was identified sharing the same structural features as Mena (mammalian enabled) and VASP, although its functionality seems to be different from that of the other members. Here we demonstrate that SEMA6A-1/Sema6A-1 is colocalized with EVL via its zyxin-like carboxyl-terminal domain that contains a modified binding motif, which further stresses the existence of functional differences between EVL and Mena/VASP. In addition these findings suggest a completely new role for transmembranous semaphorins such as SEMA6A-1/Sema6A-1 in retrograde signaling.

Assessment of differential gene expression patterns in human colon cancers

OBJECTIVE

To use a novel genomic approach to determine differential gene expression patterns in colon cancers of different metastatic potential.

SUMMARY BACKGROUND DATA

Colorectal cancer is the third leading cause of cancer deaths in the United States; despite aggressive treatment strategies, the 5-year survival rate for metastatic cancer has not changed in 50 years. The analysis of changes in gene expression patterns associated with metastasis may provide new treatment strategies.

METHODS

Human colon cancer cells KM12C (derived from a Dukes B colon cancer), KML4A (a metastatic variant derived from KM12C), and KM20 (derived from a Dukes D colon cancer) were extracted for RNA. In addition, RNA was extracted from normal colon, primary cancer, and liver metastasis in a patient with metastatic colon cancer. Gene expression patterns for approximately 1,200 human genes were analyzed and compared by cDNA array techniques.

RESULTS

Of the roughly 1,200 genes assessed in the KM cell lines, 9 genes were noted to have a more than threefold change in expression (either increased or decreased) in the more metastatic KML4A and KM20 cells compared with KM12C. Assessment of tissues from a patient with metastatic colon cancer demonstrated a more than threefold change in the expression of 14 genes in the primary cancer and liver metastasis compared with normal mucosa.

CONCLUSIONS

Using cDNA expression array technology, the authors identified genes with expression levels that are altered with metastasis. The ability to analyze and compare the expression patterns of multiple genes simultaneously provides a powerful technique to identify potential molecular targets for novel therapeutic strategies.

Characterization of the interaction between zyxin and members of the Ena/vasodilator-stimulated phosphoprotein family of proteins

Zyxin contains a proline-rich N-terminal domain that is similar to the C-terminal domain in the ActA protein of the bacteria, Listeria monocytogenes. We screened the entire amino acid sequence of human zyxin for Mena-interacting peptides and found that, as with ActA, proline-rich sequences were the sole zyxin sequences capable of binding to Ena/vasodilator-stimulated phosphoprotein (VASP) family members in vitro. From this information, we tested zyxin mutants in which the proline-rich sequences were altered. The reduction in Mena/VASP binding was confirmed by peptide tests, immunoprecipitation, and ectopic expression of zyxin variants at the surface of mitochondria. By transfection assays we showed that zyxin interaction with Mena/VASP in vivo enhances the production of actin-rich structures at the apical surface of cells. Microinjection into cells of peptides corresponding to the first proline-rich sequence of zyxin caused the loss of Mena/VASP from focal contacts. Furthermore, these peptides reduced the degree of spreading of cells replated after trypsinization. We conclude that zyxin and proteins that harbor similar proline-rich repeats contribute to the positioning of Mena/VASP proteins. The positioning of Ena/VASP family members appears to be important when the actin cytoskeleton is reorganized, such as during spreading.

Antisperm antibodies associated with infertility: properties and encoding genes of target antigens

Infertility among couples of reproductive age is a perplexing condition when the cause is indeterminate. These cases are classified as unexplained infertility. In a subset of subjects, antisperm antibodies with sperm agglutinating and/or immobilizing activities have been detected in the blood or fluids of the reproductive tract. These cases are designated as immunologic infertility although a cause and effect relationship of the antibodies to infertility has not been established. In this review, seven target sperm antigens to antibodies associated with infertility and their encoding genes are described. The antisperm antibodies (ASAs) examined were obtained from infertile women or were monoclonal antibodies (mAb) raised against human sperm proteins. All the ASAs studied possessed potent sperm agglutinating and/or immobilizing activities. The target antigens were isolated from human and other mammalian sperm, and the encoding genes identified. The seven antigens are YWK-II, BE-20, rSMP-B, BS-63 (nucleoporin-related), BS-17 (calpastatin), HED-2 (zyxin), and 75- kDa. Each antigen is a distinct and separate entity and is produced by different cells of the reproductive tract, (e.g., germ cells, epididymal epithelial cells, and Sertoli cells). No single predominant target component has been found to interact with the ASAs. It is proposed that immunologic infertility is the consequence of the combined actions of multiple ASAs in immobilizing and/or agglutinating spermatozoa, blocking spermegg interaction, preventing implantation, and/or arresting embryo development.

Zyxin, a regulator of actin filament assembly, targets the mitotic apparatus by interacting with h-warts/LATS1 tumor suppressor

The mitotic apparatus plays a pivotal role in dividing cells to ensure each daughter cell receives a full set of chromosomes and complement of cytoplasm during mitosis. A human homologue of the Drosophila warts tumor suppressor, h-warts/LATS1, is an evolutionarily conserved serine/threonine kinase and a dynamic component of the mitotic apparatus. We have identified an interaction of h-warts/LATS1 with zyxin, a regulator of actin filament assembly. Zyxin is a component of focal adhesion, however, during mitosis a fraction of cytoplasmic-dispersed zyxin becomes associated with h-warts/LATS1 on the mitotic apparatus. We found that zyxin is phosphorylated specifically during mitosis, most likely by Cdc2 kinase, and that the phosphorylation regulates association with h-warts/LATS1. Furthermore, microinjection of truncated h-warts/LATS1 protein, including the zyxin-binding portion, interfered with localization of zyxin to mitotic apparatus, and the duration of mitosis of these injected cells was significantly longer than that of control cells. These findings suggest that h-warts/LATS1 and zyxin play a crucial role in controlling mitosis progression by forming a regulatory complex on mitotic apparatus.

Fyn-binding protein (Fyb)/SLP-76-associated protein (SLAP), Ena/vasodilator-stimulated phosphoprotein (VASP) proteins and the Arp2/3 complex link T cell receptor (TCR) signaling to the actin cytoskeleton

T cell receptor (TCR)-driven activation of helper T cells induces a rapid polarization of their cytoskeleton towards bound antigen presenting cells (APCs). We have identified the Fyn- and SLP-76-associated protein Fyb/SLAP as a new ligand for Ena/ vasodilator-stimulated phosphoprotein (VASP) homology 1 (EVH1) domains. Upon TCR engagement, Fyb/SLAP localizes at the interface between T cells and anti-CD3-coated beads, where Evl, a member of the Ena/VASP family, Wiskott-Aldrich syndrome protein (WASP) and the Arp2/3 complex are also found. In addition, Fyb/SLAP is restricted to lamellipodia of spreading platelets. In activated T cells, Fyb/SLAP associates with Ena/VASP family proteins and is present within biochemical complexes containing WASP, Nck, and SLP-76. Inhibition of binding between Fyb/SLAP and Ena/VASP proteins or WASP and the Arp2/3 complex impairs TCR-dependent actin rearrangement, suggesting that these interactions play a key role in linking T cell signaling to remodeling of the actin cytoskeleton.

LPP, an actin cytoskeleton protein related to zyxin, harbors a nuclear export signal and transcriptional activation capacity

The LPP gene is the preferred translocation partner of the HMGIC gene in a subclass of human benign mesenchymal tumors known as lipomas. Here we have characterized the LPP gene product that shares 41% of sequence identity with the focal adhesion protein zyxin. LPP localizes in focal adhesions as well as in cell-to-cell contacts, and it binds VASP, a protein implicated in the control of actin organization. In addition, LPP accumulates in the nucleus of cells upon treatment with leptomycin B, an inhibitor of the export factor CRM1. The nuclear export of LPP depends on an N-terminally located leucine-rich sequence that shares sequence homology with well-defined nuclear export signals. Moreover, LPP displays transcriptional activation capacity, as measured by GAL4-based assays. Altogether, these results show that the LPP protein has multifunctional domains and may serve as a scaffold upon which distinct protein complexes are assembled in the cytoplasm and in the nucleus.

Molecular dissection of zyxin function reveals its involvement in cell motility

Spatially controlled actin filament assembly is critical for numerous processes, including the vectorial cell migration required for wound healing, cell- mediated immunity, and embryogenesis. One protein implicated in the regulation of actin assembly is zyxin, a protein concentrated at sites where the fast growing ends of actin filaments are enriched. To evaluate the role of zyxin in vivo, we developed a specific peptide inhibitor of zyxin function that blocks its interaction with alpha-actinin and displaces it from its normal subcellular location. Mislocalization of zyxin perturbs cell migration and spreading, and affects the behavior of the cell edge, a structure maintained by assembly of actin at sites proximal to the plasma membrane. These results support a role for zyxin in cell motility, and demonstrate that the correct positioning of zyxin within the cell is critical for its physiological function. Interestingly, the mislocalization of zyxin in the peptide-injected cells is accompanied by disturbances in the distribution of Ena/VASP family members, proteins that have a well-established role in promoting actin assembly. In concert with previous work, our findings suggest that zyxin promotes the spatially restricted assembly of protein complexes necessary for cell motility.

Zyxin redistributes without upregulation in migrating human keratinocytes during wound healing

Cell migration, growth, and survival is modulated by focal adhesions linking extracellular matrix proteins, cell adhesion molecules, and the cytoskeleton. Zyxin is a focal adhesion phosphoprotein that shares homology with Listeria ActA protein in promoting actin filament assembly; it also has specialized protein-protein interface domains implicating an important role in cell growth and differentiation. We investigated the distribution of zyxin in normal and migrating human keratinocytes in wounds in vitro and in situ using confocal laser microscopy. Zyxin expression in high-density nonmigrating keratinocytes versus low-density migrating keratinocytes was determined by western immunoblotting and time lapse image analysis. In normal epidermis, zyxin exhibited a punctate staining pattern throughout the cytoplasm and was excluded from the intercellular spaces. In wounds, the punctate staining also localized in the edge of the migrating keratinocyte sheets; however, intercellular spaces were absent. Likewise, in vitro keratinocytes showed punctate staining throughout the cytoplasm. Migrating cultured keratinocytes next to wounds, however, had large focal contacts in the cell periphery where actin bundles converged at focal adhesions. Western immunoblots and confocal experiments with protein synthesis inhibition by cycloheximide confirmed that this difference in distribution of zyxin in migrating versus nonmigrating keratinocytes is due to the redistribution and not upregulation of zyxin. The abundance of zyxin and its relative change in distribution from normal to migrating keratinocytes in wounds is consistent with its role in cytoskeletal organization of actin bundles.

ZRP-1, a zyxin-related protein, interacts with the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E

Protein-protein interactions play an important role in the specificity of cellular signaling cascades. By using the yeast two-hybrid system, a specific interaction was identified between the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E and a novel protein, which was termed ZRP-1 to indicate its sequence similarity to the Zyxin protein family. The mRNA encoding this protein is distributed widely in human tissues and contains an open reading frame of 1428 base pairs, predicting a polypeptide of 476 amino acid residues. The deduced protein displays a proline-rich amino-terminal region and three double zinc finger LIM domains at its carboxyl terminus. The specific interaction of this novel protein with the second PDZ domain of hPTP1E was demonstrated both in vitro, using bacterially expressed proteins, and in vivo, by co-immunoprecipitation studies. Deletion analysis indicated that an intact carboxyl terminus is required for its interaction with the second PDZ domain of hPTP1E in the yeast two-hybrid system and suggested that other sequences, including the LIM domains, also participate in the interaction. The genomic organization of the ZRP-1 coding sequence is identical to that of the lipoma preferred partner gene, another Zyxin-related protein, suggesting that the two genes have evolved from a recent gene duplication event.

An alpha-actinin binding site of zyxin is essential for subcellular zyxin localization and alpha-actinin recruitment

The LIM domain protein zyxin is a component of adherens type junctions, stress fibers, and highly dynamic membrane areas and appears to be involved in microfilament organization. Chicken zyxin and its human counterpart display less than 60% sequence identity, raising concern about their functional identity. Here, we demonstrate that human zyxin, like the avian protein, specifically interacts with alpha-actinin. Furthermore, we map the interaction site to a motif of approximately 22 amino acids, present in the N-terminal domain of human zyxin. This motif is both necessary and sufficient for alpha-actinin binding, whereas a downstream region, which is related in sequence, appears to be dispensable. A synthetic peptide comprising human zyxin residues 21-42 specifically binds to alpha-actinin in solid phase binding assays. In contrast to full-length zyxin, constructs lacking this motif do not interact with alpha-actinin in blot overlays and fail to recruit alpha-actinin in living cells. When zyxin lacking the alpha-actinin binding site is expressed as a fusion protein with green fluorescent protein, association of the recombinant protein with stress fibers is abolished, and targeting to focal adhesions is grossly impaired. Our results suggest a crucial role for the alpha-actinin-zyxin interaction in subcellular zyxin localization and microfilament organization.

The LIM-only protein PINCH directly interacts with integrin-linked kinase and is recruited to integrin-rich sites in spreading cells

PINCH is a widely expressed and evolutionarily conserved protein comprising primarily five LIM domains, which are cysteine-rich consensus sequences implicated in mediating protein-protein interactions. We report here that PINCH is a binding protein for integrin-linked kinase (ILK), an intracellular serine/threonine protein kinase that plays important roles in the cell adhesion, growth factor, and Wnt signaling pathways. The interaction between ILK and PINCH has been consistently observed under a variety of experimental conditions. They have interacted in yeast two-hybrid assays, in solution, and in solid-phase-based binding assays. Furthermore, ILK, but not vinculin or focal adhesion kinase, has been coisolated with PINCH from mammalian cells by immunoaffinity chromatography, indicating that PINCH and ILK associate with each other in vivo. The PINCH-ILK interaction is mediated by the N-terminal-most LIM domain (LIM1, residues 1 to 70) of PINCH and multiple ankyrin (ANK) repeats located within the N-terminal domain (residues 1 to 163) of ILK. Additionally, biochemical studies indicate that ILK, through the interaction with PINCH, is capable of forming a ternary complex with Nck-2, an SH2/SH3-containing adapter protein implicated in growth factor receptor kinase and small GTPase signaling pathways. Finally, we have found that PINCH is concentrated in peripheral ruffles of cells spreading on fibronectin and have detected clusters of PINCH that are colocalized with the alpha5beta1 integrins. These results demonstrate a specific protein recognition mechanism utilizing a specific LIM domain and multiple ANK repeats and suggest that PINCH functions as an adapter protein connecting ILK and the integrins with components of growth factor receptor kinase and small GTPase signaling pathways.

Visualization and molecular analysis of actin assembly in living cells

Actin filament assembly is critical for eukaryotic cell motility. Arp2/3 complex and capping protein (CP) regulate actin assembly in vitro. To understand how these proteins regulate the dynamics of actin filament assembly in a motile cell, we visualized their distribution in living fibroblasts using green flourescent protein (GFP) tagging. Both proteins were concentrated in motile regions at the cell periphery and at dynamic spots within the lamella. Actin assembly was required for the motility and dynamics of spots and for motility at the cell periphery. In permeabilized cells, rhodamine-actin assembled at the cell periphery and at spots, indicating that actin filament barbed ends were present at these locations. Inhibition of the Rho family GTPase rac1, and to a lesser extent cdc42 and RhoA, blocked motility at the cell periphery and the formation of spots. Increased expression of phosphatidylinositol 5-kinase promoted the movement of spots. Increased expression of LIM-kinase-1, which likely inactivates cofilin, decreased the frequency of moving spots and led to the formation of aggregates of GFP-CP. We conclude that spots, which appear as small projections on the surface by whole mount electron microscopy, represent sites of actin assembly where local and transient changes in the cortical actin cytoskeleton take place.

Analysis of a human cDNA containing a tissue-specific alternatively spliced LIM domain

A unique clone, isolated from a human pancreatic cDNA library, was sequenced and characterized. Northern blot analysis showed that the gene is active in a number of fetal and adult tissues, and immunoblots showed expression in nuclear and cytosolic cell fractions. The gene corresponding to the clone was localized to chromosome 13 by human/rodent somatic cell hybrid panels. The largest open reading frame contains a LIM domain, and the deduced peptide from the open reading frame appears to have the characteristics of a LIM-only protein, designated LMO7. RT-PCR and genomic sequence analyses indicate that expression of this gene product is subject to tissue-specific modulation by elimination of the LIM domain by alternative splicing in neural tissues.

Expression of reciprocal fusion transcripts of the HMGIC and LPP genes in parosteal lipoma

Parosteal lipomas are rare benign neoplasms of adipose tissue that exhibit a contiguous relationship with the periosteum. These lipomas of the bone share some histopathologic features with their commonly occurring soft tissue counterparts. The latter are well-characterized cytogenetically, primarily by rearrangements involving chromosome region 12q13-q15. In particular, translocations involving 12q13-q15 are prominent, with chromosomal region 3q27-q28 as the most frequent translocation partner. Recently, we established that the genes HMGIC at 12q15 and LPP at 3q27-28 are affected by the 3;12-translocation and demonstrated that, as a direct result, HMGIC/LPP and LPP/HMGIC fusion transcripts are expressed in soft tissue lipomas. In this study, cytogenetic and molecular analyses revealed similar findings in a parosteal lipoma. Specifically, a t(3;12)(q28;q14) was detected cytogenetically in a parosteal lipoma from a 51-year-old female and subsequently confirmed by FISH utilizing a chromosome 3 breakpoint spanning YAC probe and chromosome 12 breakpoint flanking cosmid probes. RT-PCR analysis showed expression of HMGIC/LPP and LPP/HMGIC fusion transcripts in this parosteal lipoma; nucleotide sequence analysis revealed that these transcripts are identical to those expressed in soft tissue lipomas characterized by a 3;12-translocation. These findings lend further support to a common histopathogenesis between lipomas of soft tissue and parosteal origin.

UNC-115, a conserved protein with predicted LIM and actin-binding domains, mediates axon guidance in C. elegans

Axon guidance receptors modulate the growth cone cytoskeleton through signaling pathways that are not well understood. Here, we describe the C. elegans unc-115 gene, which encodes a candidate cytoskeletal linker protein that acts in axon guidance. unc-115 mutants have defects in a subset of axons, particularly as the affected axons change environments during outgrowth. The unc-115 gene encodes a putative actin-binding protein that is similar to the human actin-binding protein abLIM/limatin; it has a villin headpiece domain and three LIM domains that could mediate protein interactions. unc-115 is expressed in neurons during their development and is required cell-autonomously in certain neurons for normal axon guidance. We propose that UNC-115 modulates the growth cone actin cytoskeleton in response to signals received by growth cone receptors.

LIM domains of cysteine-rich protein 1 (CRP1) are essential for its zyxin-binding function

Previous studies have demonstrated that the adhesion-plaque protein, zyxin, interacts specifically with a 23 kDa protein, called the cysteine-rich protein 1 (CRP1), which has been implicated in myogenesis. Primary sequence analyses have revealed that both zyxin and CRP1 exhibit multiple copies of a structural motif called the LIM domain. LIM domains, which are defined by the consensus CX2CX16-23HX2CX2CX2CX16-23CX2-3(C,H,D), are found in a variety of proteins that are involved in cell growth and differentiation. Recent studies have established that LIM domains are zinc-binding structures that mediate specific protein-protein interactions. For example, in the case of the zyxin-CRP1 interaction, one of zyxin's three LIM domains is necessary and sufficient for binding to CRP1. Because the CRP1 molecule is comprised primarily of two LIM domains, we were interested in the possibility that the binding site for zyxin on CRP1 might also be contained within a single LIM domain. Consistent with the hypothesis that the LIM domains of CRP1 are critical for the protein's zyxin-binding function, zinc-depleted CRP1 displays a reduced zyxin-binding activity. However, domain mapping analyses have revealed that neither of the two individual LIM domains of CRP1 can support a wild-type interaction with zyxin. Collectively, our results suggest that the binding site for zyxin on CRP1 is not contained within a single contiguous sequence of amino acids. Instead, the interaction appears to rely on the co-ordinate action of a number of residues that are displayed in both of CRP1's LIM domains.

The human TRIP6 gene encodes a LIM domain protein and maps to chromosome 7q22, a region associated with tumorigenesis

The thyroid receptor interacting protein-6 (TRIP6) was first identified as a ligand-dependent binding partner for the thyroid hormone receptor in a yeast two-hybrid screen. A partial TRIP6 cDNA clone that was isolated in the initial screen encodes two copies of the LIM domain. The LIM domain is a double zinc-finger structure that mediates protein-protein interactions. Here we report the complete amino acid sequence of human TRIP6. The TRIP6 protein displays a proline-rich N-terminal region linked to three tandemly arrayed C-terminal LIM domains. The global molecular architecture and sequence of TRIP6 place it in the same family as the adhesion plaque protein, zyxin, and the lipoma preferred partner (LPP). Zyxin and LPP are implicated in cellular signaling and tumorigenesis, respectively. By radiation hybrid mapping, the human TRIP6 gene was assigned to a segment of chromosome 7q22 that is commonly deleted in malignant myeloid diseases and uterine leiomyoma.

Molecular cloning and characterization of FHL2, a novel LIM domain protein preferentially expressed in human heart

A full-length cDNA clone encoding a novel LIM-only protein was isolated and sequenced from a human fetal heart cDNA library. This full-length clone consists of 1416 base pairs and has a predicted open reading frame (ORF) encoding 279 amino acids. The ORF of this polypeptide codes for the human heart-specific four and a half LIM-only protein 2 (FHL2). It possesses an extra zinc finger that is a half LIM domain and four repeats of LIM domain. When the human FHL2 cDNA probe was used to hybridize with poly-A RNA of various human tissues, a very strong signal could be seen in heart tissues, and only moderately low signals could be detected in placenta, skeletal muscle and ovary. Virtually no signal could be detected in brain, lung, liver, kidney, pancreas, spleen, thymus, prostate, testis, small intestine, colon or peripheral blood leukocyte. FHL2 was mapped to chromosome 2q12-q13 by fluorescent in-situ hybridization (FISH).

Using the yeast two-hybrid system to identify human epithelial cell proteins that bind gonococcal Opa proteins: intracellular gonococci bind pyruvate kinase via their Opa proteins and require host pyruvate for growth

Neisseria gonorrhoeae opacity-associated (Opa) proteins are a family of outer membrane proteins involved in gonococcal adherence to and invasion of human cells. We wanted to identify additional roles for Opa in the infectious process and used the yeast two-hybrid system to identify human epithelial cell proteins that interact with Opa proteins. Although this system has been used successfully to identify many types of interacting proteins, it has not been used to screen a human cell cDNA library for binding partners of a prokaryotic outer membrane protein. Therefore, we were also interested in exploring the versatility of the yeast two-hybrid system in identifying bacteria-host interactions. Using OpaP from strain F62SF as bait, we screened a HeLa cell cDNA library for Opa-interacting proteins (OIPs). We identified five different OIPs, designated OIP1-OIP5, two of which are homologous to human proteins--thyroid hormone receptor interacting protein (TRIP6) and pyruvate kinase isoenzyme M2 (PK). In the studies presented here, we investigated the interaction between Opa proteins and PK in more depth. Opa-PK interactions were confirmed by in vitro and in vivo assays independent of the yeast two-hybrid system. Escherichia coli expressing six different Opa proteins from gonococcal strain FA1090 all bound more PK than Opa-negative E. coli in in vitro binding assays. Using anti-PK antibody and fluorescence microscopy, we showed that human epithelial cell PK co-localizes with intracellular Opa+ gonococci and E. coli expressing Opa proteins. Using a mutant of N. gonorrhoeae unable to grow on pyruvate or lactate, it appears that intracellular pyruvate is essential for gonococcal growth and survival. These results suggest a novel mechanism in bacterial pathogenesis, i.e. the requirement for direct molecular interaction with a host metabolic enzyme (PK) for the acquisition of an essential intracellular carbon source and growth substrate (pyruvate). These results demonstrate that the yeast two-hybrid system is a valuable tool for identifying biologically relevant interactions between bacteria and host proteins, providing valuable leads for further investigations into novel mechanisms of bacterial pathogenesis.

Zyxin: zinc fingers at sites of cell adhesion

Zyxin is a low abundance phosphoprotein that is localized at sites of cell-substratum adhesion in fibroblasts. Zyxin displays the architectural features of an intracellular signal transducer. The protein exhibits an extensive proline-rich domain, a nuclear export signal and three copies of the LIM motif, a double zinc-finger domain found in many proteins that play central roles in regulation of cell differentiation. Zyxin interacts with alpha-actinin, members of the cysteine-rich protein (CRP) family, proteins that display Src homology 3 (SH3) domains and Ena/VASP family members. Zyxin and its partners have been implicated in the spatial control of actin filament assembly as well as in pathways important for cell differentiation. Based on its repertoire of binding partners and its behavior, zyxin may serve as a scaffold for the assembly of multimeric protein machines that function in the nucleus and at sites of cell adhesion.

Comparison of three members of the cysteine-rich protein family reveals functional conservation and divergent patterns of gene expression

Members of the cysteine-rich protein (CRP) family are evolutionarily conserved proteins that have been implicated in the processes of cell proliferation and differentiation. In particular, one CRP family member has been shown to be an essential regulator of cardiac and skeletal muscle development. Each of the three vertebrate CRP isoforms characterized to date is composed of two copies of the zinc-binding LIM domain with associated glycine-rich repeats. In this study, we have addressed the biological significance of the CRP multigene family by comparing the subcellular distributions, biochemical properties, and expression patterns of CRP1, CRP2, and CRP3/MLP. Our data reveal that all three CRP family members, when expressed in adherent fibroblasts, associate specifically with the actin cytoskeleton. Moreover, all three CRP isoforms are capable of interacting with the cytoskeletal proteins alpha-actinin and zyxin. Together, these observations suggest that CRP family members may exhibit overlapping cellular functions. Differences between the three CRPs are evident in their protein expression patterns in chick embryos. CRP1 expression is detected in a variety of organs enriched in smooth muscle. CRP2 is restricted to arteries and fibroblasts. CRP3/MLP is dominant in organs enriched in striated muscle. CRP isoform expression is also developmentally regulated in the chick. Our findings suggest that the three CRP family members perform similar functions in different muscle derivatives. The demonstration that all members of the CRP family are associated with cytoskeletal components that have been implicated in the assembly and organization of filamentous actin suggests that CRPs contribute to muscle cell differentiation via effects on cytoarchitecture.

Nuclear-cytoplasmic shuttling of the focal contact protein, zyxin: a potential mechanism for communication between sites of cell adhesion and the nucleus

Integrin-dependent cell adhesion to specific extracellular matrix molecules has been demonstrated to trigger dramatic changes in gene expression that can affect cell fate. However, little is understood about the molecular mechanism by which events at sites of cell- substratum adhesion are communicated to the cell interior to regulate the transcriptional apparatus. By analogy to classical mechanisms of cell surface receptor function, it seems likely that some components of the integrin-activated signal transduction machinery will be colocalized with cell adhesion molecules. Zyxin is a low abundance phosphoprotein that accumulates with integrins at sites of cell-substratum attachment. Here we show that zyxin exhibits a functional nuclear export signal that is required to keep zyxin concentrated in the cytoplasm and is sufficient to direct nuclear proteins to the cytosol. Furthermore, we demonstrate that native zyxin shuttles between the nucleus and sites of cell adhesion in fibroblasts and is thus an excellent candidate for relaying information between these two compartments.

A novel proline-rich motif present in ActA of Listeria monocytogenes and cytoskeletal proteins is the ligand for the EVH1 domain, a protein module present in the Ena/VASP family

The ActA protein of the intracellular pathogen Listeria monocytogenes induces a dramatic reorganization of the actin-based cytoskeleton. Two profilin binding proteins, VASP and Mena, are the only cellular proteins known so far to bind directly to ActA. This interaction is mediated by a conserved module, the EVH1 domain. We identify E/DFPPPPXD/E, a motif repeated 4-fold within the primary sequence of ActA, as the core of the consensus ligand for EVH1 domains. This motif is also present and functional in at least two cellular proteins, zyxin and vinculin, which are in this respect major eukaryotic analogs of ActA. The functional importance of the novel protein-protein interaction was examined in the Listeria system. Removal of EVH1 binding sites on ActA reduces bacterial motility and strongly attenuates Listeria virulence. Taken together we demonstrate that ActA-EVH1 binding is a paradigm for a novel class of eukaryotic protein-protein interactions involving a proline-rich ligand that is clearly different from those described for SH3 and WW/WWP domains. This class of interactions appears to be of general importance for processes dependent on rapid actin remodeling.