CrkL is an adapter for Wiskott-Aldrich syndrome protein and Syk

Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia are caused by mutations of the WAS protein (WASP) gene. WASP may be involved in the regul ation of podosome, an actin-rich dynamic cell adhesion structure formed by various types of cells. The molecular links between WASP and podosomes or o ther cell adhesion structures are unknown. Platelets express an SH2-SH3 ada pter molecule, CrkL, that can directly associate with paxillin, which is lo calized in podosomes, The hypothesis that CrkL binds to WASP was, therefore , tested. Results from coprecipitation experiments using anti-CrkL and GST- fusion proteins suggest that CrkL binds to WASP through its SH3 domain and that the binding was not affected by WASP tyrosine phosphorylation. The bin ding of GST-fusion SH3 domain of PSTPIP1 in vitro was also not affected by WASP tyrosine phosphorylation, suggesting that the binding of the SH3 domai ns to WASP is not inhibited by tyrosine phosphorylation of WASP. Anti-CrkL also coprecipitates a 72-kd protein, which was identified as syk tyrosine k inase, critical for collagen induced-platelet activation. CrkL immunoprecip itates contain kinase-active syk, as evidenced by an in vitro kinase assay. Coprecipitation experiments using GST-fusion CrkL proteins suggest that bo th SH2 and SH3 domains of CrkL are involved in the binding of CrkL to syk, WASP, CrkL, syk, and paxillin-like Hic-5 incorporated to platelet cytoskele ton after platelet aggregation. Thus, CrkL is a novel molecular adapter for WASP and syk and may potentially transfer these molecules to the cytoskele ton through association with cytoskeletal proteins such as Hic-5. (Blood, 2 001; 97:2633-2639) (C) 2001 by The American Society of Hematology.