Introduction

Abstract

In cells growing under normal conditions, proteins are continuously produced, transported to cell compartments where they express their functions, and assembled into complexes if necessary, after which they function for indicated periods and finally are consumed. Tertiary structure of protein appears to be thermodynamically determined, but the process of protein folding is still a matter of dispute. It has become clear, however, that immature secretory proteins are retained in endoplasmic reticulum by forming complexes with stress proteins functioning as molecular chaperones. Recently, a body of evidence has accumulated that folding of nascent polypeptides is transiently prevented also by forming complexes with constitutively expressed isoforms of stress proteins. The complexes are not dissociated until their translations are completed, assembled and, if indicated, transported into membraneous organella. Molecular chaperones assist their substrates in the complexes in folding, assembly and appropriate processing. In addition, dissociation of supramolecular complexes such as clathrin coats occurs only in the presence of molecular chaperones. Finally, degradation of proteins also has been reported to occur in a stress protein-dependent manner. Thus, it is evident that cellular proteins are taken care of by stress proteins from the cradle to the grave. This part deals with all of these problems.