HEAT-SHOCK PROTEINS - MOLECULAR CHAPERONES OF PROTEIN BIOGENESIS

Heat shock proteins (Hsps) were first identified as proteins whose syn thesis was enhanced by stresses such as an increase in temperature. Re cently, several of the major Hsps have been shown to be intimately inv olved in protein biogenesis through a direct interaction with a wide v ariety of proteins. As a reflection of this role, these Hsps have been referred to as molecular chaperones. Hsp70s interact with incompletel y folded proteins, such as nascent chains on ribosomes and proteins in the process of translocation from the cytosol into mitochondria and t he endoplasmic reticulum. Hsp60 also binds to unfolded proteins, preve nting aggregation and facilitating protein folding. Although less well defined, other Hsps such as Hsp90 also play important roles in modula ting the activity of a number of proteins. The function of the proteol ytic system is intertwined with that of molecular chaperones. Several components of this system, encoded by heat-inducible genes, are respon sible for the degradation of abnormal or misfolded proteins. The buddi ng yeast Saccharomyces cerevisiae has proven very useful in the analys is of the role of molecular chaperones in protein maturation, transloc ation, and degradation. In this review, results of experiments are dis cussed within the context of experiments with other organisms in an at tempt to describe the current state of understanding of these ubiquito us and important proteins.