The bonds that tie: catalyzed disulfide bond formation.

The pioneering studies of Anfinsen on ribonuclease have guided much of the research in the field of protein folding (Anfinsen et al., 1961). Anfinsen showed that, in vitro, reduced and denatured ribonuclease could refold into active enzyme with the formation of the appropriate disulfide bonds. These findings demonstrated that there was sufficient information in the primary amino acid sequence of the protein to guide folding and that, in the appropriate redox environment, the proper disulfide bonds formed. However, the rates and efficiency of protein folding and disulfide bond formation in the test tube are often much worse than those found in the cell. Anfinsen himself subsequently discovered protein disulfide isomerase (PDI), a catalyst of oxidative folding. Despite this indication that auxiliary factors could guide protein folding, the persistent impression remained until recentlythat the folding process and disulfide bond formation occur in vivo without the need for catalysis. Two classes of proteins that assist in the protein folding process have been described. Molecular chaperones are thought to act by preventing improper interactions leading to aggregation and other events taking a protein off the productive folding pathway. Chaperones thus do not actually catalyze folding. However, catalysts have been found for two slow steps in protein folding, cis-trans prolyl isom-erization and disulfide bond formation. Clear evidence for the function of prolyl isomerases as folding catalysts in vivo is still lacking. The recent isolation of mutants that are severely defective in disulfide bond formation has, however, confirmed that disulfide bond formation is facilitated in vivo. A substantial body of in vitro evidence had previously implicated PDI in the catalysis of disulfide bond formation and rearrangement (Creighton et al., 1980; Freedman et al., 1989). The description, in this issue of Cell, of yeast mutants in PDI that fail to form disulfides in carboxypepti-dase Y directly demonstrates the importance of PDI in the process of disulfide bond formation in vivo (LaMantia and Lennarz, 1993 [this issue of Cc/d). In prokaryotes, mutations in the d.sbA gene show a dramatic and pleiotropic decrease in the rate of disulfide bond formation in secreted proteins (Bardwell et al., 1991). The proteins affected range from normal Escherichia coli constituents such as alkaline phosphatase and OmpA to the cloned eukaryotic proteins, urokinase and tissue plasminogen activator. It is now clear that the formation of disulfide bonds is a cata-lyzed process in both eukaryotes and prokaryotes. Disulfide-linked protein folding can involve both the …