Elucidation of the physiological roles of DOCK180 family proteins in platelets and megakaryocytes.

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Platelets play a critical role in normal hemostasis and pathological thrombosis like myocardial infarction. Their functions are regulated by complex signal transduction pathways, which, in turn, are targets of anti-platelet drugs. Thus, the studies on signal transduction in platelets are important for clinical medicine. Rac, a small molecular G-protein, is essential for several platelet functions. GTP-bound rac is functionally active, while GDP-bound one is inactive. Rac-Guanine nucleotide exchange factor (rac-GEF) is responsible for activation of rac. Surprisingly, little is known of rac-GEF in platelets. In this study, we found that platelets express two rac-GEFs, DOCK180 and DOCK5, in addition to previously identified Vav1. DOCK180 is a well characterized prototype of DOCK180 family GEF proteins and a bona fide rac-GEF. DOCK5 is closely related to DOCK180, but has been poorly characterized. Using specific antibodies, we found that there are two different isoforms of human DOCK5. One is expressed in platelets and Hela cells. The other is expressed in A549 cells and not in platelets. When forcedly expressed, in COS7 cells, DOCK5 is indeed a specific rac-GEF. Immunofluorescent studies revealed dot-like distribution of DOCK5 in the cytoplasm of spreading platelets. DOCK180 is known to bind to several proteins like crk and ELMO1. We found that the same is true for DOCK5. Thus, I have set the basis for further studies for DOCK180 family rac-GEF in platelets and megakaryocytes, since platelets proteins are mostly carried over from the precursor cells.