Folding and assembly of viral membrane proteins.

It is now clear that folding in the ER is a dynamic, energy-driven process involving a host of cellular folding enzymes and molecular chaperones (see Fig. 1). Within this high-capacity folding environment, nascent molecules fold quickly and efficiently, while misfolded proteins are recognized and retained, being either degraded or rescued. The quality control mechanisms which account for this selective retention are most likely redundant and general in nature--an almost innumerable number of structures, from both endogenous and exogenous proteins, are operated on with equal efficiency. Studies with viral membrane proteins will continue to help illuminate these processes and have contributed greatly to the concepts of conformational maturation and quality control. Furthermore, while the effects of mutations on protein structure and transport cannot always be predicted, useful generalizations can now be made to help develop experimental strategies. Future studies will have to address a variety of unresolved issues. Given the almost limitless sequence and structural variability exhibited by proteins which fold in the ER, no one molecular chaperone is likely to be able to bind to all folding intermediates. Thus, GRP78-BiP is likely to be only one of a number of resident ER molecular chaperones. Identifying these molecules, the structural features to which they bind, and how they interact with other components of the folding machinery are areas in which important advances can be made. A particularly intriguing problem concerns the mechanisms by which the folding machinery is regulated. The synthesis of GRP78-BiP, for example, is strongly induced by elevated levels of misfolded proteins in the ER. How the levels of misfolded molecules are monitored and how this information can be used to regulate GRP78-synthesis is not known. Likewise, the means by which the ER environment, such as its oxidizing potential, is regulated have yet to be elucidated. It is important to note that a direct role for GRP78-BiP (or any other ER molecular chaperone) in folding has yet to be demonstrated in vitro. Reconstituting complex folding reactions in vitro will provide a way to specifically address the roles of folding enzymes and chaperones in protein folding and assembly. The molecular mechanisms which lead to the retention of misfolded proteins in the ER are still poorly understood, as are the mechanisms which lead to their degradation. Finally, whether quality control mechanisms play significant roles in regulating protein transport in other organelles represents an interesting area of research.