Abstract: The present invention relates to a new protein, convertin I, which belongs to a family of mammalian convertase proteins characterized by partial amino acid sequence homology to each of the following: the precursor processing endoprotease of yeast KEX2, bacterial subtilisins, and the human fur gene product, furin. The protein which is a subject of this invention is further characterized as a mammalian convertase, which is capable of hydrolytically cleaving, for example, prohormones to yield active hormones. Specifically, convertin I is capable of cleaving peptide bonds following certain sequences of two adjacent basic amino acids. Convertin I has a molecular weight of approximately 70 kd by electrophoresis. Another aspect of this invention is a recombinant vector, incorporating the coding sequences which are the objects of this invention. Methods of preparing the endoprotease protein, including molecular engineering methods are described.