HEMATOPOIETIC-CELL PHOSPHATASE, SHP-1, IS CONSTITUTIVELY ASSOCIATED WITH THE SH2 DOMAIN-CONTAINING LEUKOCYTE PROTEIN, SLP-76, IN B-CELLS

Src homology region 2 (SH2) domain-containing phosphatase 1 (SHP-1; pr eviously named HCP, PTP1C, SH-PTP1, and SHP) is a cytosolic protein ty rosine phosphatase that contains two SH2 domains. Recent data have dem onstrated that the gene encoding SHP-1 is mutated in motheaten (me) an d viable motheaten (me(v)) mice resulting in autoimmune disease. More recently, SHP-1 has been shown to negatively regulate B cell antigen r eceptor (BCR)-initiated signaling. To elucidate potential mechanisms o f SHP-1 action in BCR signal transduction, we studied proteins that in teract with SHP-1 in B cells. Both anti-SHP-1 antibody and the two SH2 domains of SHP-1 expressed as glutathione S-transferase fusion protei ns precipitated at least three phosphoproteins of similar to 75, 110, and 50 kD upon anti-immunoglobulin M stimulation of the WEHI-231 immat ure B cell line. Binding of SHP-1 to the 75- and 110-kD proteins appea l ed to be mediated mainly by the NH2-terminal SH2 domain of SHP-1, wh ereas both the NH2- and COOH-terminal SH2 domains are required for max imal binding to the 150-kD protein. Immunoprecipitation and Western bl ot analysis revealed that the SHP-1-associated 73-kD protein is the he matopoietic cell-specific, SH2-containing protein SLP-76. Further, thi s protein-protein association was constitutively observed and stable d uring the early phase of BCR signaling. However, significant tyrosine phosphorylation of SLP-76 as well as of SHP-1 was observed after BCR l igation. Constitutive association of SHP-1 with SLP-76 could also be d etected ill normal splenic B cells. Collectively, these results sugges t possible mechanisms by which SHP-1 may modulate signals delivered by BCR engagement.