Proteolytic cleavage of an Insulin-like Growth Factor Binding Protein (IGFBP) by a novel serine protease that contains an IGFBP-like domain

Insulin-like growth factor binding proteins (IGFBPs) play an important role in the biology of the insulin-like growth factors. These roles are regulated in part by cellular proteases that can selectively cleave the IGFBPs. Despite much work in this area over the last several years, the identity of the protease(s) that carry out these cleavages has remained unclear. Recently, a putative serine protease was identified that also encodes an IGFBP-like domain (Zumbrunn, J. & Trueb, B. FEBS Letters 398: (1996) pp.187-192). We have begun to characterize the role of this protease in the IGF pathway by expressing it in an insect cell expression system, followed by purification using antibody affinity chromatography. We show that the recombinant protease (IGFBPase) is active and that it selectively cleaves one of six known IGFBP's using both western and ligand blotting approaches. The proteolytic activity of IGFBPase was found to be inhibited by the serine protease inhibitors PMSF and 3,4-dichloroisocoumerin, as well as by bivalent metal ions such as Zinc. Expression analysis of IGFBPase in a number of tissue culture cell lines correlates with the ability of these cell lines to cleave the IGFBP. Bioinformatic analysis indicates that IGFBPase is but one member of this novel family of mammalian serine proteases.