Macrophage stimulating protein (MSP) binds to its receptor via the MSP β chain

Abstract

Macrophage stimulating protein (MSP) is a 78-kDa disulfide-linked heterodimer belonging to the plasminogen-related kringle protein family. MSP activates the RON receptor protein-tyrosine kinase, which results in cell migration, shape change, or proliferation. A structure-activity study of MSP was performed using proMSP, MSP, MSP α and β chains, and a complex including the first two kringles and IgG Fc (MSP-NK2). Radioiodinated MSP and MSP β chain both bound specifically to RON. The K(d) of 1.4 nM for MSP β chain is higher than the reported K(d) range of 0.6-0.8 nM for MSP. Pro-MSP, MSP α chain, and MSP-NK2 did not bind. Only MSP stimulated RON autophosphorylation. Although the β chain bound to RON and partially inhibited MSP-induced RON phosphorylation in kidney 293 cells, it did not induce RON phosphorylation. Pro-MSP, MSP α chain, or MSP-NK2 failed to activate RON, consistent with their inability to bind to the RON receptor. Functional studies showed that only MSP induced cell migration, and shape change in resident macrophages, and growth of murine keratinocytes. Our data indicate that the primary receptor binding domain is located in a region of the MSP β chain, in contrast to structurally similar hepatocyte growth factor, in which the receptor binding site is in the α chain. However, full activation of RON requires binding of the complete MSP disulfide-linked αβ chain heterodimer.