Proprotein convertase models based on the crystal structures of furin and kexin: Explanation of their specificity

In eukaryotes, many secreted proteins and peptide hormones are excised from larger precursors by calcium-dependent serine proteinases, the proprotein/prohormone convertases, (PCs). These PCs cleave their protein substrates very specifically following multiple basic residues. The seven mammalian PCs and their yeast orthologue kexin are multi-domain proteinases consisting of a subtilisin-related catalytic domain, a conserved P-domain and a variable, often cysteine-rich domain, which in some PCs is followed by an additional C-terminal trans-membrane domain and a short cytoplasmic domain. The recently published crystal structures of the soluble mouse furin and yeast kexin ectodomains have revealed the relative arrangement of catalytic and P domains, the exact domain fold and the detailed architecture of the substrate binding clefts. Based on these experimental structures, we now have modelled the structures of the other human/mouse PCs. According to topology and to structure-based sequence comparisons, these other PCs closely resemble furin, with PC4, PACE4 and PC5/6 being more similar, and PC1/3, PC2 and PC7 being less similar to furin. Except for PC1 and PC2, this order of similarity is valid for the catalytic as well as for the P domains, and is almost reversed using kexin as a reference molecule. A similar order results from the number and clustering of negative charges lining the non-prime subsites, explaining the gradually decreasing requirement for basic residues N-terminal to substrate cleavage sites. The preference of the different PCs for distinct substrates seems to be governed by overall charge compensation and matching of the detailed charge distribution pattern. (C) 2004 Elsevier Ltd. All rights reserved.