The cloning, genomic organization and expression of the focal contact protein paxillin in Drosophila

Evolution and Expression of Paxillin Genes in Teleost Fish

Background

Paxillin family proteins regulate intracellular signaling downstream of extracellular matrix adhesion. Tissue expression patterns and cellular functions of Paxillin proteins during embryo development remain poorly understood. Additionally, the evolution of this gene family has not been thoroughly investigated.

Results

This report characterizes the evolution and expression of a novel Paxillin gene, called Paxillin-b, in Teleosts. Alignments indicate that Teleost Paxillin-a and Paxillin-b proteins are highly homologous to each other and to human Paxillin. Phylogenetic and synteny analyses suggest that these genes originated from the duplication of an ancestral Paxillin gene that was in a common ancestor of Teleosts and Tetrapods. Analysis of the spatiotemporal expression profiles of Paxillin-a and Paxillin-b using zebrafish revealed both overlapping and distinct domains for Paxillin-a and Paxillin-b during embryo development. Localization of zebrafish Paxillin orthologs expressed in mammalian cells demonstrated that both proteins localize to focal adhesions, similar to mammalian Paxillin. This suggests these proteins regulate adhesion-dependent processes in their endogenous tissues.

Conclusion

Paxillin-a and Paxillin-b were generated by duplication in Teleosts. These genes likely play similar roles as Paxillin genes in other organisms. This work provides a framework for functional investigation of Paxillin family members during development using the zebrafish as an in vivo model system.

The Caenorhabditis elegans paxillin orthologue, PXL-1, is required for pharyngeal muscle contraction and for viability

Caenorhabditis elegans pxl-1 is the orthologue of vertebrate paxillin and is expressed in body wall and pharyngeal muscle. In body wall muscle PXL-1 localizes to dense bodies, M-lines, and adhesion plaques, and in pharyngeal muscle PXL-1 localizes to podosome-like actin attachment complexes. PXL-1 is required in the pharynx for muscle contraction and viability.

We have identified the gene C28H8.6 (pxl-1) as the Caenorhabditis elegans orthologue of vertebrate paxillin. PXL-1 contains the four C-terminal LIM domains conserved in paxillin across all species and three of the five LD motifs found in the N-terminal half of most paxillins. In body wall muscle, PXL-1 antibodies and a full-length green fluorescent protein translational fusion localize to adhesion sites in the sarcomere, the functional repeat unit in muscle responsible for contraction. PXL-1 also localizes to ring-shaped structures near the sarcolemma in pharyngeal muscle corresponding to podosome-like sites of actin attachment. Our analysis of a loss-of-function allele of pxl-1, ok1483, shows that loss of paxillin leads to early larval arrested animals with paralyzed pharyngeal muscles and eventual lethality, presumably due to an inability to feed. We rescued the mutant phenotype by expressing paxillin solely in the pharynx and found that these animals survived and are essentially wild type in movement and body wall muscle structure. This indicates a differential requirement for paxillin in these two types of muscle. In pharyngeal muscle it is essential for contraction, whereas in body wall muscle it is dispensable for filament assembly, sarcomere stability, and ultimately movement.

PTP-H2 and PTP-H3 from Microplitis demolitor Bracovirus localize to focal adhesions and are antiphagocytic in insect immune cells

Viruses in the family Polydnaviridae are symbiotically associated with parasitoid wasps. Wasps inject polydnaviruses (PDVs) when laying an egg into their insect host, and expression of viral gene products causes several physiological alterations, including immunosuppression, that allow the wasp's progeny to develop. As with other PDVs, most Microplitis demolitor bracovirus (MdBV) genes are related variants that form gene families. The largest MdBV gene family includes 13 members that encode predicted proteins related to protein tyrosine phosphatases (PTPs). Sequence analysis during the present study indicated that five PTP family members (PTP-H2, -H3, -N1, and -N2) have fully conserved catalytic domains, whereas other family members exhibited replacements, deletions, or rearrangements of amino acids considered essential for tyrosine phosphatase activity. Expression studies indicated that most MdBV PTP genes are expressed in virus-infected host insects, with transcript abundance usually being highest in hemocytes. MdBV-infected hemocytes also exhibited higher levels of tyrosine phosphatase activity than noninfected hemocytes. We produced expression constructs for four of the most abundantly expressed PTP family members and conducted functional studies with hemocyte-like Drosophila S2 cells. These experiments suggested that recombinant PTP-H2 and PTP-H3 are functional tyrosine phosphatases whereas PTP-H1 and PTP-J1 are not. PTP-H2 and -H3 localized to focal adhesions in S2 cells, and coexpression with another MdBV gene product, Glc1.8, resulted in complete inhibition of phagocytosis.

Regulation of Rho and Rac signaling to the actin cytoskeleton by paxillin during Drosophila development

Paxillin is a prominent focal adhesion docking protein that regulates cell adhesion and migration. Although numerous paxillin-binding proteins have been identified and paxillin is required for normal embryogenesis, the precise mechanism by which paxillin functions in vivo has not yet been determined. We identified an ortholog of mammalian paxillin in Drosophila (Dpax) and have undertaken a genetic analysis of paxillin function during development. Overexpression of Dpax disrupted leg and wing development, suggesting a role for paxillin in imaginal disc morphogenesis. These defects may reflect a function for paxillin in regulation of Rho family GTPase signaling as paxillin interacts genetically with Rac and Rho in the developing eye. Moreover, a gain-of-function suppressor screen identified a genetic interaction between Dpax and cdi in wing development. cdi belongs to the cofilin kinase family, which includes the downstream Rho target, LIM kinase (LIMK). Significantly, strong genetic interactions were detected between Dpax and Dlimk, as well as downstream effectors of Dlimk. Supporting these genetic data, biochemical studies indicate that paxillin regulates Rac and Rho activity, positively regulating Rac and negatively regulating Rho. Taken together, these data indicate the importance of paxillin modulation of Rho family GTPases during development and identify the LIMK pathway as a critical target of paxillin-mediated Rho regulation.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

Expression of the LIM proteins paxillin and Hic-5 in human tissues

The LIM domain is a protein-protein interaction motif critically involved in a variety of fundamental biological processes, including cytoskeletal organization, cell lineage specification, and organ development. In this study we examined the expression of the LIM proteins paxillin and Hic-5 in adult human tissues by immunohistochemistry and immunoblotting. Paxillin expression was widespread and observed both in non-muscle and muscle tissues. Of the latter, paxillin was mainly expressed in multinuclear striated muscle. In contrast, Hic-5 showed restricted expression and was expressed in muscle tissues, mainly in mononuclear smooth muscle. Taken together with previous findings, it appears likely that the counterbalance between paxillin and Hic-5 may be deeply involved in muscle differentiation.

The paxillin LD motifs

Adapter/scaffold proteins, through their multidomain structure, perform a fundamental role in facilitating signal transduction within cells. Paxillin is a focal adhesion adapter protein implicated in growth factor- as well as integrin-mediated signaling pathways. The amino-terminus of paxillin contains five leucine-rich sequences termed LD motifs. These paxillin LD motifs are highly conserved between species as well as within the paxillin superfamily. They mediate interactions with several structural and regulatory proteins important for coordinating changes in the actin cytoskeleton associated with cell motility and cell adhesion as well as in the regulation of gene expression.

Molecular dissection of actopaxin-integrin-linked kinase-Paxillin interactions and their role in subcellular localization

Paxillin is a focal adhesion adapter protein involved in integrin signaling. We have recently reported that the paxillin LD1 motif acts as a binding interface for both the actin-binding protein actopaxin and the serine/threonine integrin-linked kinase (ILK). In this report we demonstrate the direct association between actopaxin and ILK and dissect the role of the respective interactions in their subcellular localization. Co-immunoprecipitation experiments were employed to map the binding sites on ILK and actopaxin. ILK binds to the CH2 domain of actopaxin. However, an actopaxin CH2 domain mutant defective for paxillin binding (paxillin binding subdomain mutant) retains the capacity to bind ILK, indicating that paxillin and ILK binding sites on actopaxin are distinct. Actopaxin binds to the C terminus of ILK. Despite the direct binding between actopaxin and ILK, mutation analysis confirmed a primary role for paxillin in their localization to focal adhesions. Interestingly, an ILK mutant (E359K) that was previously reported to act as dominant negative for ILK function was unable to bind actopaxin or paxillin and failed to localize to focal adhesions. This mutant also exhibited in vitro kinase activity comparable with wild-type ILK. Taken together, these data suggest that normal ILK signaling is dependent on efficient localization involving multiple protein interactions.

Paxillin null embryonic stem cells are impaired in cell spreading and tyrosine phosphorylation of focal adhesion kinase

Paxillin is a focal-adhesion associated protein implicated in the regulation of integrin signaling and organization of the actin cytoskeleton. Paxillin associates with numerous signaling molecules including adaptor molecules (p130Cas, CRK), kinases (FAK, Pyk2, PAK and SRC), tyrosine phosphatases (PTP-PEST), ARF-GAP proteins (p95pkl, PAG3) and papillomavirus E6 oncoproteins. Although paxillin is tyrosine phosphorylated in cellular processes such as cell attachment and spreading, little direct evidence is available about paxillin's role in these events. Targeted gene disruption was used to generate paxillin null mouse embryonic stem (ES) cells and paxillin null differentiated cells. Paxillin null ES cells exhibit delayed spreading on integrin binding substrates fibronectin and laminin, and there is reduced tyrosine phosphorylation of Focal Adhesion Kinase (FAK). Both of these phenotypes are recovered in paxillin knockout cells upon exogenous re-expression of paxillin. The individual LD motifs of paxillin that are binding sites for FAK, vinculin and ARF-GAP proteins, as well as tyrosine residues that when phosphorylated create binding sites for CRK family members, are dispensable for FAK phosphorylation and early cell spreading. These results demonstrate that paxillin contributes to attachment-dependent tyrosine phosphorylation of FAK and early cell spreading in ES cells.

Paxillin: a focal adhesion-associated adaptor protein

Paxillin is a focal adhesion-associated, phosphotyrosine-containing protein that may play a role in several signaling pathways. Paxillin contains a number of motifs that mediate protein-protein interactions, including LD motifs, LIM domains, an SH3 domain-binding site and SH2 domain-binding sites. These motifs serve as docking sites for cytoskeletal proteins, tyrosine kinases, serine/threonine kinases, GTPase activating proteins and other adaptor proteins that recruit additional enzymes into complex with paxillin. Thus paxillin itself serves as a docking protein to recruit signaling molecules to a specific cellular compartment, the focal adhesions, and/or to recruit specific combinations of signaling molecules into a complex to coordinate downstream signaling. The biological function of paxillin coordinated signaling is likely to regulate cell spreading and motility.

A novel muscle LIM-only protein is generated from the paxillin gene locus in Drosophila

Paxillin is a protein containing four LIM domains, and functions in integrin signaling. We report here that two transcripts are generated from the paxillin gene locus in Drosophila; one encodes a protein homolog of the vertebrate Paxillin (DPxn37), and the other a protein with only three LIM domains, partly encoded by its own specific exon (PDLP). At the myotendinous junctions of Drosophila embryos where integrins play important roles, both DPxn37 and PDLP are highly expressed with different patterns; DPxn37 is predominantly concentrated at the center of the junctions, whereas PDLP is highly enriched at neighboring sides of the junction centers, primarily expressed in the mesodermal myotubes. Northern blot analysis revealed that DPxn37 is ubiquitously expressed throughout the life cycle, whereas PDLP expression exhibits a biphasic pattern during development, largely concomitant with muscle generation and remodeling. Our results collectively reveal that a unique system exists in Drosophila for the generation of a novel type of LIM-only protein, highly expressed in the embryonic musculature, largely utilizing the Paxillin LIM domains.