Subtleties among subtilases. The structural biology of Kex2 and furin-related prohormone convertases.

Abstract

Prohormone processing was discovered 35 years ago with the observation that proinsulin is the precursor of both the A and the B chains of insulin (Steiner, 1969). During the next 15 years, many candidate proteases were proposed and rejected before genetic and biochemical evidence identified specific processing proteases in Saccharomyces cerevisiae that are involved in α‐factor biosynthesis (Julius et al ., 1983, 1984). The initial cleavages of the precursor—endoproteolytic scissions carboxyl to Lys‐Arg sites—are due to the action of Kex2 protease. Mammalian Kex2 homologues, known as furin, PC1/3, PC2, PC4, PACE4, PC5/6 and PC7/LPC were cloned throughout the 1990s. They were shown to process hormone and neuropeptide precursors, growth factors, receptors, metallo‐ and aspartyl proteases, envelope glycoproteins of many viruses including HIV, and bacterial toxins such as anthrax‐protective antigen (Zhou et al ., 1999; Thomas, 2002). These Kex2‐homologous prohormone convertases (PCs) are unrelated to the tryptic enzymes of the blood coagulation cascade that hydrolyse precursors carboxyl to basic amino acids. By contrast, PCs are related to degradative enzymes, such as subtilisin and proteinase K (Fuller et al ., 1989). Thus, accounting for the refined functions and specificity of PCs has been a major research problem. Molecular‐genetic studies have established a ‘P‐domain’ carboxyl to the subtilisin domain that is essential for the biosynthesis and/or activity of PCs. Biochemical analyses have further indicated …