MACROPHAGE STIMULATING PROTEIN: MECHANISM OF ACTIVATION AND RECEPTOR DIMERIZATION MODEL

Maria Miller1, Alla Danilkovitch2 and Edward J. Leonard2

1 Macromolecular Structure Laboratory, ABL-BRP, NCI-Frederick Cancer Research and Development Center; Frederick, MD 21702, USA (e-mail: millerm@ncifcrf.gov)
2 Laboratory of Immunobiology, NCI-Frederick Cancer Research and Development Center; Frederick, MD 21702, USA

Macrophage stimulating protein (MSP) activates the RON receptor protein-tyrosine kinase and promotes cell migration, shape change and proliferation. MSP and its close homologue, hepatocyte growth factor (HGF), form a distinct family of growth factors that evolved from plasminogen. They have the same domain organization as plasminogen and like plasminogen are converted to their mature form by cleavage to disulfide-linked ,a, b-chain heterodimers, even though their b-chain serine protease like domains are devoid of enzymatic activity due to mutations in the catalytic triad. Surprisingly, despite structural similarities, the high affinity receptor binding regions are different: a chain for HGF [1] and b chain for MSP [2].

We carried out modeling and site-directed mutagenesis studies of the MSP b chain domain and identified an arginine residue critical for the receptor binding ability of MSP. Based on the comparison with HGF we propose: (i) MSP is bivalent with respect to receptor binding, and a monomeric ligand induces receptor dimerization and activation. The second, low affinity, binding site is located within the region corresponding to the primary site of HGF, i.e. in the MSP a chain. The complex may be stabilized by interactions between receptor pairs. (ii) HGF binding is the converse of MSP; the primary receptor binding determinant is situated within the chain in a region with a high density of positively charged residues; the secondary site on the b chain is cryptic. (iii) The site located in the b chain serine protease-like domain becomes exposed only after proteolytic cleavage of single chain precursors to the biologically active, mature form of growth factors.

Research sponsored in part by the National Cancer Institute, DHHS, under contract with ABL.

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