The e S-Sence of -SH in the ER

The eSSence of-SH Minireview in the ER for the abundant ER protein Grp94/gp96, a stress-regulated member of the Hsp90 family, has yet to be identified. The response to accumulation of misfolded proteins is the induction of the unfolded protein responses, characterized by increased expression of ER resident Boston, Massachusetts 02115 chaperones and PDI (Cox et al., 1993). The failure to successfully complete protein folding is thus linked to the cell's attempt to repair this defect. In conclusion, Most proteins can reach their native folded state sponta-the environment into which a nascent chain makes its neously in the test tube, but do so slowly and ineffi-first appearance is quite different from the conditions ciently. In contrast, protein folding in the living cell is used to establish requirements for, and patterns of, S-S remarkably fast and efficient, owing to the guidance bond formation in the course of refolding in vitro. provided by molecular chaperones. Clearly not all S-S bond formation must occur cotrans-In eukaryotes, protein folding and S-S bond formation lationally: imposition of a reducing environment on the are coupled processes that occur both co-and postER , by addition of a reducing agent such as DTT to the translationally in the endoplasmic reticulum (ER), the culture medium, abrogates S-S bond formation almost main port of entry for secretory and membrane proteins. instantly, yet the effects of such treatment are largely Once formed, S-S bonds are considered part of the reversible in the absence of ongoing protein synthesis primary structure of a protein and often maintain the (Braakman et al., 1992). Similarly, in vitro translation stable folding pattern of proteins that contain them. Two experiments indicate that S-S bond formation is initiated papers in the January issue of Molecular Cell (Frand and on nascent chains, but may be completed posttransla-Kaiser, 1998; Pollard et al., 1998) shed new light on the tionally, strongly dependent on the location of the pro-molecular machinery that introduces S-S bonds in vivo. spective S-S bond in the completely folded structure. Much of our present understanding of protein folding Polypeptides synthesized in vitro under reducing condi-and the role of S-S bonds therein stems from experi-tions can acquire S-S bonds posttranslationally simply ments that examine the refolding in vitro of completely by changing the redox potential of the folding environ-reduced, full length polypeptides. However, a nascent ment. Refolding experiments in vitro have established polypeptide chain must acquire not only the proper …