Getting Newly Synthesized Proteins into Shape

• Published in: Cell 2000-04, Vol.101 (2), p.119-122
• Main Authors: Bukau, Bernd, Deuerling, Elke, Pfund, Christine, Craig, Elizabeth A
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Cytosol - chemistry; Escherichia coli; Eukaryotic Cells - chemistry; Molecular Chaperones - chemistry; Protein Folding; Semliki Forest virus
• ISSN: 0092-8674; 1097-4172
• DOI: 10.1016/S0092-8674(00)80806-5

Cellular synthesis of polypeptides is an amazing, complex, and efficient process. An E. coli cell utilizes up to 20,000 ribosomes to produce an estimated total of 30,000 polypeptides per minute. Yet it is only the beginning. Each newly made protein must be folded into its correct tertiary structure. How this is achieved is one of the most basic, and complicated, questions of molecular biology. When does the process of folding begin in the lifetime of a protein? Crystallographic data of bacterial ribosomes identified a peptide exit tunnel in the large subunit with a length of 100 Ae and an average diameter of about 20 Ae. This tunnel is long enough to accommodate extended chains of about 30 residues and perhaps longer peptides in helical conformation, but certainly not peptides with tertiary structure. Once the N-terminal residues are synthesized, the nascent polypeptide emerges into the crowded environment of the cytosol. It is well established that nascent proteins can fold cotranslationally (i.e., while still attached to ribosomes) in cell-free translation systems. Recently, a study using the Semliki Forest virus capsid protein established that cotranslational folding can occur in living bacterial and mammalian cells. In both prokaryotic and eukaryotic cells, the N-terminal domain of this protein folds rapidly into its native state during translation, well before termination of synthesis. Such cotranslational stepwise folding may be beneficial to the folding process since it avoids unwanted collisions between domains during the vulnerable early stages of folding. Specialized proteins called molecular chaperones, which bind nonnative states of proteins, are found in all species. Work in past years established that chaperones facilitate the refolding of proteins denatured in the cytosol due to heat or chemical stresses. An understanding of the role of chaperones in assisting folding of newly synthesized cytosolic proteins has been more elusive. This review focuses on the function of chaperones in the folding of polypeptides as they emerge from the ribosome and are released into the cytosol. Coimmunoprecipitation of extracts from pulse-labeled cells has allowed an estimate of the number of proteins interacting with a particular chaperone in vivo. Aggregation of newly synthesized proteins in strains carrying mutations in chaperone genes has been taken as evidence of the in vivo role for a particular chaperone. Together with the analysis of folding of specifi