Actopaxin, a new focal adhesion protein that binds paxillin LD motifs and actin and regulates cell adhesion.

Paxillin is a focal adhesion adapter protein involved in the integration of growth factor- and adhesion-mediated signal transduction pathways. Paxillin LD motifs have been demonstrated to bind to several proteins associated with remodeling of the actin cytoskeleton including the focal adhesion kinase, vinculin, and a complex of proteins comprising p95PKL, PIX, and PAK (Turner, C.E., M. C. Brown, J.A. Perrotta, M.C. Riedy, S.N. Nikolopoulos, A.R. McDonald, S. Bagrodia, S. Thomas, and P.S. Leventhal. 1999. J. Cell Biol. 145:851-863). In this study, we report the cloning and initial characterization of a new paxillin LD motif-binding protein, actopaxin. Analysis of the deduced amino acid sequence of actopaxin reveals a 42-kD protein with two calponin homology domains and a paxillin-binding subdomain (PBS). Western blotting identifies actopaxin as a widely expressed protein. Actopaxin binds directly to both F-actin and paxillin LD1 and LD4 motifs. It exhibits robust focal adhesion localization in several cultured cell types but is not found along the length of the associated actin-rich stress fibers. Similar to paxillin, it is absent from actin-rich cell-cell adherens junctions. Also, actopaxin colocalizes with paxillin to rudimentary focal complexes at the leading edge of migrating cells. An actopaxin PBS mutant incapable of binding paxillin in vitro cannot target to focal adhesions when expressed in fibroblasts. In addition, ectopic expression of the PBS mutant and/or the COOH terminus of actopaxin in HeLa cells resulted in substantial reduction in adhesion to collagen. Together, these results suggest an important role for actopaxin in integrin-dependent remodeling of the actin cytoskeleton during cell motility and cell adhesion.

The LD4 motif of paxillin regulates cell spreading and motility through an interaction with paxillin kinase linker (PKL).

The small GTPases of the Rho family are intimately involved in integrin-mediated changes in the actin cytoskeleton that accompany cell spreading and motility. The exact means by which the Rho family members elicit these changes is unclear. Here, we demonstrate that the interaction of paxillin via its LD4 motif with the putative ARF-GAP paxillin kinase linker (PKL) (Turner et al., 1999), is critically involved in the regulation of Rac-dependent changes in the actin cytoskeleton that accompany cell spreading and motility. Overexpression of a paxillin LD4 deletion mutant (paxillinDeltaLD4) in CHO.K1 fibroblasts caused the generation of multiple broad lamellipodia. These morphological changes were accompanied by an increase in cell protrusiveness and random motility, which correlated with prolonged activation of Rac. In contrast, directional motility was inhibited. These alterations in morphology and motility were dependent on a paxillin-PKL interaction. In cells overexpressing paxillinDeltaLD4 mutants, PKL localization to focal contacts was disrupted, whereas that of focal adhesion kinase (FAK) and vinculin was not. In addition, FAK activity during spreading was not compromised by deletion of the paxillin LD4 motif. Furthermore, overexpression of PKL mutants lacking the paxillin-binding site (PKLDeltaPBS2) induced phenotypic changes reminiscent of paxillinDeltaLD4 mutant cells. These data suggest that the paxillin association with PKL is essential for normal integrin-mediated cell spreading, and locomotion and that this interaction is necessary for the regulation of Rac activity during these events.

Paxillin LD4 motif binds PAK and PIX through a novel 95-kD ankyrin repeat, ARF-GAP protein: A role in cytoskeletal remodeling.

Paxillin is a focal adhesion adaptor protein involved in the integration of growth factor- and adhesion-mediated signal transduction pathways. Repeats of a leucine-rich sequence named paxillin LD motifs (Brown M.C., M.S. Curtis, and C.E. Turner. 1998. Nature Struct. Biol. 5:677-678) have been implicated in paxillin binding to focal adhesion kinase (FAK) and vinculin. Here we demonstrate that the individual paxillin LD motifs function as discrete and selective protein binding interfaces. A novel scaffolding function is described for paxillin LD4 in the binding of a complex of proteins containing active p21 GTPase-activated kinase (PAK), Nck, and the guanine nucleotide exchange factor, PIX. The association of this complex with paxillin is mediated by a new 95-kD protein, p95PKL (paxillin-kinase linker), which binds directly to paxillin LD4 and PIX. This protein complex also binds to Hic-5, suggesting a conservation of LD function across the paxillin superfamily. Cloning of p95PKL revealed a multidomain protein containing an NH2-terminal ARF-GAP domain, three ankyrin-like repeats, a potential calcium-binding EF hand, calmodulin-binding IQ motifs, a myosin homology domain, and two paxillin-binding subdomains (PBS). Green fluorescent protein- (GFP-) tagged p95PKL localized to focal adhesions/complexes in CHO.K1 cells. Overexpression in neuroblastoma cells of a paxillin LD4 deletion mutant inhibited lamellipodia formation in response to insulin-like growth fac- tor-1. Microinjection of GST-LD4 into NIH3T3 cells significantly decreased cell migration into a wound. These data implicate paxillin as a mediator of p21 GTPase-regulated actin cytoskeletal reorganization through the recruitment to nascent focal adhesion structures of an active PAK/PIX complex potentially via interactions with p95PKL.

The human non-muscle alpha-actinin protein encoded by the ACTN4 gene suppresses tumorigenicity of human neuroblastoma cells.

alpha-Actinins are actin-binding proteins important in organization of the cytoskeleton and in cell adhesion. We have cloned and characterized a cDNA from human neuroblastoma cell variants which encodes the second non-muscle alpha-actinin isoform designated ACTN4 (actinin-4). mRNA encoded by the ACTN4 gene, mapped to chromosome 4, is abundant in non-tumorigenic, substrate-adherent human neuroblastoma cell variants but absent or only weakly expressed in malignant, poorly substrate-adherent neuroblasts. It is also present in many adherent tumor cell lines of diverse tissue origins. Cell lines typically co-express ACTN4 and ACTN1, a second non-muscle alpha-actinin gene. Expression is correlated with substrate adhesivity. Analysis of deduced amino acid sequences suggests that the two isoforms may differ in function and in regulation by calcium. Moreover, ACTN4 exhibits tumor suppressor activity. Stable clones containing increased levels of alpha-actinin, isolated from highly malignant neuroblastoma stem cells [BE(2)-C] after transfection with a full-length ACTN4 cDNA, show decreased anchorage-independent growth ability, loss of tumorigenicity in nude mice, and decreased expression of the N-myc proto-oncogene.

Isolation and characterization of the human secretogranin II gene promoter.

The goal of this study was to isolate and functionally characterize the human secretogranin II (SgII) gene promoter. SgII is a member of the granin family of proteins which are selectively expressed in neurosecretory cells. The human SgII promoter contains a consensus TATA box and cyclic AMP response element (CRE) 35 and 74 bp upstream of the transcription start site, respectively, elements also found in the mouse and rat SgII gene promoters. Transfection studies showed that 869 bp of the human SgII promoter were sufficient to confer cell type-specific expression of an SgII promoter-luciferase reporter gene in neurosecretory PC-12, GH and BE(2)-M17 cells. The activity of the human SgII promoter was also compared in three N-type, human neuroblastoma cell lines [BE(2)-M17, SMS-KAN and SH-SY5Y], which differ markedly in the level of SgII expression. SgII promoter activities in the neuroblastoma cell lines correlated not only with the levels of SgII but also the levels of the cyclic AMP response element-binding protein CREB which were highest in BE(2)-M17 cells and lowest in SH-SY5Y cells. To establish that the activity of the human SgII promoter in these neuroblastoma cell lines is dependent on the level of CREB, rat CREB was overexpressed in SH-SY5Y cells. SgII promoter activity was up to 8-fold higher in SH-SY5Y cells overexpressing CREB. These results suggest that SgII expression is a marker for neuronal differentiation in human neuroblastoma cell lines and is dependent on the level of CREB expression.

Integrin-linked kinase (ILK) binding to paxillin LD1 motif regulates ILK localization to focal adhesions.

Paxillin is a focal adhesion adapter protein involved in integrin signaling. Paxillin LD motifs bind several focal adhesion proteins including the focal adhesion kinase, vinculin, the Arf-GTPase-activating protein paxillin-kinase linker, and the newly identified actin-binding protein actopaxin. Microsequencing of peptides derived from a 50-kDa paxillin LD1 motif-binding protein revealed 100% identity with integrin-linked kinase (ILK)-1, a serine/threonine kinase that has been implicated in integrin, growth factor, and Wnt signaling pathways. Cloning of ILK from rat smooth muscle cells generated a cDNA that exhibited 99.6% identity at the amino acid level with human ILK-1. A monoclonal antibody raised against a region of the carboxyl terminus of ILK, which is identical in rat and human ILK-1 protein, recognized a 50-kDa protein in all cultured cells and tissues examined. Binding experiments showed that ILK binds directly to the paxillin LD1 motif in vitro. Co-immunoprecipitation from fibroblasts confirmed that the association between paxillin and ILK occurs in vivo in both adherent cells and cells in suspension. Immunofluorescence microscopy of fibroblasts demonstrated that endogenous ILK as well as transfected green fluorescent protein-ILK co-localizes with paxillin in focal adhesions. Analysis of the deduced amino acid sequence of ILK identified a paxillin-binding subdomain in the carboxyl terminus of ILK. In contrast to wild-type ILK, paxillin-binding subdomain mutants of ILK were unable to bind to the paxillin LD1 motif in vitro and failed to localize to focal adhesions. Thus, paxillin binding is necessary for efficient focal adhesion targeting of ILK and may therefore impact the role of ILK in integrin-mediated signal transduction events.