ACCURATE AND EFFICIENT CLEAVAGE OF THE HUMAN INSULIN PRORECEPTOR BY THE HUMAN PROPROTEIN-PROCESSING PROTEASE FURIN - CHARACTERIZATION AND KINETIC-PARAMETERS USING THE PURIFIED, SECRETED SOLUBLE PROTEASE EXPRESSED BY A RECOMBINANT BACULOVIRUS

Maturation of the insulin proreceptor in a late Golgi compartment requ ires cleavage at an Arg-Lys-Arg-Arg processing site, suggesting involv ement of furin, a transmembrane serine protease of the Kex2 family of processing enzymes. A genetically engineered secreted, soluble form of human furin (ss-furin), expressed by infection of insect cells with a recombinant baculovirus, was purified to near homogeneity. ss-furin e xhibited rapid and efficient cleavage of both isoforms of the human in sulin proreceptor in solubilized extracts of cultured mammalian cells expressing preproreceptor cDNA. Proreceptor cleavage occurred at the p hysiological processing site as judged by the effects of mutations in this site on cleavage by purified ss-furin. Moreover, purified ss-furi n exhibited specificity for proreceptor cleavage identical to that of the endogenous insulin proreceptor-processing enzyme. Furin thus displ ays the properties expected of an insulin proreceptor processing enzym e in that it (i) cleaves the proreceptor efficiently and at the correc t site; (ii) exhibits the same specificity in processing variant prore ceptors as the endogenous enzyme; (iii) appears to be localized in the correct secretory compartment; and (iv) has the same broad pattern of tissue distribution as the insulin proreceptor.