The murine stk gene product, a transmembrane protein tyrosine kinase, is a receptor for macrophage-stimulating protein

Macrophage-stimulating protein (MSP) was originally identified as an inducer of murine resident peritoneal macrophage responsiveness to chemoattractants. We recently showed that the product of RON, a protein tyrosine kinase cloned from a human keratinocyte library, is the receptor for MSP. Similarity of murine stk to RON led us to determine if the stk gene product is the murine receptor for MSP. Radiolabeled MSP could bind to NIH 3T3 cells transfected with murine stk cDNA (3T3/stk). Binding was saturable and was inhibited by unlabeled MSP but not by structurally related proteins, including hepatocyte growth factor and plasminogen. Specific binding to STK was demonstrated by cross-linking of ¹²⁵I-labeled MSP to membrane proteins of 3T3/stk cells, which resulted in a protein complex with a molecular mass of 220 kDa. This radiolabeled complex comprised ¹²⁵I-MSP and STK, since it could be immunoprecipitated by antibodies to the STK β chain. Binding of MSP to stk cDNA-transfected cells induced tyrosine phosphorylation of the 150- kDa STK β chain within I min and caused increased motile activity. These results establish the murine stk gene product as a specific transmembrane protein tyrosine kinase receptor for MSP. Inasmuch as the stk cDNA was cloned from a hematopoietic stem cell, our data suggest that in addition to macrophages and keratinocytes, a cell in the hematopoietic lineage may also be a target for MSP.