REGULATION OF INTEGRIN-MEDIATED P130(CAS) TYROSINE PHOSPHORYLATION INHUMAN B-CELLS - A ROLE FOR P59(FYN) AND SHP2

Engagement of beta 1 integrins in terminally differentiated human B ce ll lines, such as ARH-77, leads to prominent tyrosine phosphorylation of the p130 Crk-associated substrate (Cas), Cas regulates the assembly of several SH2 and SH3 domain-containing proteins into signaling comp lexes, which are potentially involved in the propagation of downstream signals. Ne demonstrate here that immunoprecipitated Cas from beta 1 integrin-stimulated ARH-77 cells was associated sith tyrosine kinase a nd phosphatase activities and that integrin ligation led to the recrui tment of at least p59(Fyn) tyrosine kinase and SHP2 tyrosine phosphata se in Cas immune complexes, Cotransfection studies in COS-7 cells furt her indicated that Fyn/Cas physical interaction and Fyn-mediated Cas p hosphorylation required amino acids 638-889 in the C-terminal region o f Gas. This sequence contains both c-Src SH2 and SNS domain-binding mo tifs. In vitro binding studies using glutathione S-transferase fusion proteins derived from the SK2 or SH3 domains of Fyn suggested that bot h Fyn domains can participate in Fyn/Cas interaction. These data impli cate Fyn and SHP2 as potential modulators of Cas signaling complexes i n B cells.