IDENTIFICATION OF MAJOR BINDING-PROTEINS AND SUBSTRATES FOR THE SH2-CONTAINING PROTEIN-TYROSINE-PHOSPHATASE SHP-1 IN MACROPHAGES

The protein tyrosine phosphatase SHP-1 is a critical regulator of macr ophage biology, but its detailed mechanism of action remains largely u ndefined. SHP-1 associates with a 130-kDa tyrosyl-phosphorylated speci es (P130) in macrophages, suggesting that P130 might be an SHP-1 regul ator and/or substrate. Here we show that P130 consists of two transmem brane glycoproteins, which we identify as PIR-B/p91A and the signal-re gulatory protein (SIRP) family member BIT. These proteins also form se parate complexes with SHP-2. BIT, but not PIR-B, is in a complex with the colony-stimulating factor 1 receptor (CSF-1R), suggesting that BIT may direct SHP-1 to the CSF-1R. BIT and PIR-B bind preferentially to substrate-trapping mutants of SHP-1 and are hyperphosphorylated in mac rophages from motheaten viable mice, which express catalytically impai red forms of SHP-1, indicating that these proteins are SHP-1 substrate s. However, BIT and PIR-B are hypophosphorylated in motheaten macropha ges, which completely lack SHP-1 expression. These data suggest a mode l in which SHP-1 dephosphorylates specific sites on BIT and PIR-B whil e protecting other sites from dephosphorylation via its SH2 domains. F inally, BIT and PIR-B associate with two tyrosyl phosphoproteins and a tyrosine kinase activity, Tyrosyl phosphorylation of these proteins a nd the level of the associated kinase activity are increased in the ab sence of SHP-1. Our data suggest that BIT and PIR-B recruit multiple s ignaling molecules to receptor complexes, where they are regulated by SHP-1 and/or SHP-2.