The alpha2gbeta1 integrin is a collagen receptor involved in platelet activation, smooth muscle cell and T lymphocyte migration, endothelial attachment and other events in the vasculature. We previously demonstrated that collagen-induced platelet aggregation depends upon occupancy of this integrin (along with an unknown co-receptor, possible glycoprotein VI). We also found that collagen activates, in an alpha2beta1-dependent manner, the non-receptor tyrosine kinase Syk, followed by phospholipase Cgamma2, events that appear necessary for collagen-induced aggregation. How the alpha2beta1 integrin activates platelets or induces these signal transduction events is unknown. Much evidence in the integrin field suggests that the short cytoplasmic domains of integrins play critical roles in transmitting signals into cells, potentially due to regulatory molecular that bind to these domains. To identify potential cytoplasmic domain regulatory proteins, we screened a library in the yeast two-hybrid system, using the lapha2 integrin cytoplasmic domain as bait. We have identified a partial cDNA sequence that encodes what appears to be an extremely interesting protein. The mRNA for this protein is large (9.5kb), widely distributed, and appears to be expressed in platelets, smooth muscle cells and other vascular and non-vascular cells. Moreover, nucleotide sequence analysis indicates a region of this protein that is highly homologous to numerous serine/threonine and dual specificity kinases, suggesting that the protein is a kinase. A separate smaller region is homologous to RasGAP, a Ras GTPase activating protein, and indeed this protein binds to Ras in the yeast two- hybrid system, but not to other control proteins, suggesting that his protein might also play a role in regulating Ras or Ras-related proteins. We propose to first complete the sequencing of the cDNA encoding this protein, and second to further characterize the protein with regard to its structure, cell and tissue distribution, and function. Characterizing of this novel protein may help us to better understand alpha2beta1-mediated in platelets and other vascular cells.
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