Production of a broad spectrum of regulatory proteins in eukaryotes occurs via an intricate cascade of biosynthetic and secretory processes. Often these proteins initially are synthesized as parts of higher molecular weight, but inactive, precursor proteins. Specific endoproteolytic processing of these proproteins is required to generate the regulatory proteins in a mature and biologically active form. Such endoproteolysis generally occurs at cleavage sites consisting of particular sequence motifs of basic amino acids, often paired basic residues. This phenomenon, first observed almost 25 years ago, has intrigued scientists ever since then. Nevertheless, the responsible enzymes remained elusive for long. The first known eukaryotic enzyme with the exquisite cleavage specificity for paired basic amino acid residues was the prohormone processing enzyme kexin (EC 3.4.21.61), a subtilisin-like serine protease that is encoded by the KEX2 gene of yeast Saccharomyces cerevisiae. Recently, a number of kexin-like mammalian proprotein-processing enzymes were discovered. The enzyme furin, which is encoded by the fur gene, was the first and can be considered the prototype of a mammalian subclass of subtilisin-like serine proteases. It is predicted to contain a "prepro" domain, a subtilisin-like catalytic domain, a middle domain, a cysteine-rich region, a transmembrane anchor, and a cytoplasmic domain. Furin is expressed in a wide variety of tissues, perhaps even in all tissues. In all likelihood, it is the enzyme responsible for the proteolytic bioactivation of a wide variety of precursor proteins. Two other novel mammalian proprotein-processing enzymes are PC1 (also known as PC3) and PC2. Some of the protein domains of these enzymes resemble those in kexin and furin, however, there are also differences. The PC1/PC3 and PC2 enzymes exhibit a more restricted expression pattern than furin. It has been suggested that PC1/PC3 and PC2 are involved primarily in the processing of prohormones within the regulated secretory pathway of cells of endocrine and neural tissue. Recently, the coding sequences for two other candidate mammalian proprotein-processing enzymes were identified. They were called PACE4 and PC4. Like that of furin, the tissue distribution of PACE4 is widespread. PC4, however, may represent a candidate for a precursor-processing endoprotease that is specifically expressed in the testicular germ cells. Finally, DNA sequences encoding kexin- and furin-like candidate pro-protein-processing enzymes have been identified in Drosophila melanogaster, Dfur1 and Dfur2 genes; in Xenopus laevis, Xen-14 gene; and in Caenorhabditis elegans, bli-4 gene.(ABSTRACT TRUNCATED AT 400 WORDS)