Leupaxin Is a Novel LIM Domain Protein That Forms a Complex with PYK2

Abstract

We have identified a novel cytoplasmic protein, leupaxin, that is preferentially expressed in hematopoietic cells and is most homologous to the focal adhesion protein, paxillin. Leupaxin possesses two types of protein interaction domains. There are four carboxyl-terminal LIM domains in leupaxin that share 70% amino acid identity and 80% similarity with those in paxillin. Paxillin LIM domains mediate localization to focal contacts. In the amino-terminal region of leupaxin there are three short stretches of approximately 13 amino acids that share 70 -90% similarity with paxillin LD motifs. Paxillin LD motifs have been implicated in focal adhesion kinase (FAK) and vinculin binding resulting in the localization of FAK to focal adhesions. Leupaxin is expressed in cell types, such as macrophage, that lack FAK. We demonstrate here that leupaxin associates with a second FAK family member, PYK2. As leupaxin and PYK2 are both preferentially expressed in leukocytes they may therefore form a cell type-specific signaling complex. We also demonstrate that leupaxin is a substrate for a tyrosine kinase in lymphoid cells and thus may function in and be regulated by tyrosine kinase activity. Leupaxin is thus a phosphotyrosine protein with LD and LIM binding motifs most homologous to paxillin that may assemble and regulate PYK2 signaling complexes in leukocytes.Cell adhesion, spreading, and migration are mediated by integrin interactions with extracellular and cell surface ligands (1, 2). In adherent cell types such as epithelial cells and fibroblasts a complex of cytoplasmic proteins localizes to the termini of actin bundles at sites of integrin-dependent close cell contact with substratum or extracellular matrix proteins (3). These complexes, designated focal adhesions, have been implicated in the regulation of cell locomotion, survival, and proliferation (4, 5).Focal adhesions are rich in tyrosine kinases and tyrosinephosphorylated proteins, which may reflect a role for tyrosine kinases in integrin signaling (6, 7). Protein-tyrosine kinases that are focal adhesion proteins includes focal adhesion kinase (FAK), 1 Src, and Src family kinases. A second FAK family member, PYK2, has been identified by several groups and designated CAK␤ (8), RAFTK (9), FAK2 (10), and CADTK (11). PYK2 and FAK are closely related in their overall structure, and both are phosphorylated on tyrosine in response to integrin engagement, T cell receptor engagement or chemokine stimulation (12-15). These stimuli all modulate integrin-dependent adhesion. PYK2 and FAK both associate with paxillin, p130cas , and Src (16,17). Although PYK2 possesses a focal adhesion targeting domain that is highly homologous with FAK, the majority of PYK2 displays a more diffuse cytoplasmic distribution with a small percent present in focal adhesions (18). This indicates that FAK and PYK2 may possess both overlapping and unique functions.Tyrosine phosphoproteins present in focal adhesions include FAK, vinculin, zyxin, and paxillin. A second paxillin family member, Hic-5, has rece...

Paxillin: Adapting to Change

“…Thus far no binding partners have been identified for LD5 or the degenerate LD3 motif. Importantly, the capacity of Hic-5 and leupaxin to interact with these LD-binding proteins has been confirmed (64,79,150,186,266,278).…”

“…Leupaxin is a 386-amino acid 45-kDa family member that is predominantly expressed in leukocytes, as is reflected in the naming convention (150). Leupaxin is located on chromosome 11cen-11q12.3 and is encoded by 9 exons.…”

Large‐scale proteomic analysis of tyrosine‐phosphorylation induced by T‐cell receptor or B‐cell receptor activation reveals new signaling pathways

“…In contrast, integrin a7 and b1 chains as well as leupaxin were detected specifically in the PY proteomes of the lymphoid cell lines. Leupaxin is preferentially expressed in hematopoietic cells and a member of the paxillin family of proteins associated with focal adhesions [26]. It was therefore not surprising that leupaxin was specifically detected in T and B cells.…”

Leupaxin acts as a mediator in prostate carcinoma progression through deregulation of p120catenin expression

“…In addition to the eponymous paxillin, the androgen receptor-associated protein 55 kDa (ARA55) and leupaxin (LPXN) belong to the paxillin superfamily (Brown and Turner, 2004). All members of the paxillin family are characterized by the presence of two protein-protein interaction domains, namely LD motifs and LIM domains (Lipsky et al, 1998). By means of this protein structure they can attach to the cytoplasmic domain of integrins and transmit signals from the outside of the cell.…”

“…Leupaxin was originally characterized as a multifunctional adaptor protein, which is preferentially expressed in hematopoietic cells (Lipsky et al, 1998). In murine osteoclasts, FAK, PTP-PEST and p95PKL (paxillin kinase linker) could be identified as interaction partners of LPXN (Gupta et al, 2003).…”