Getting Newly Synthesized Proteins Minireview into

Universitä t Freiburg with the analysis of folding of specific endogenous proteins in mutant strains, these studies have led to signifi-Hermann Herder Str. 7 D-79104 Freiburg cant advances in our understanding of protein folding in the complex cellular milieu. Germany Nascent chains emerging at the peptide exit tunnel of Madison, Wisconsin 53706 the ribosome are awaited by a welcoming committee of ribosome-associated chaperones. These factors have been most thoroughly studied in E. coli. Trigger factor (TF), which is found in all eubacteria analyzed, is the Cellular synthesis of polypeptides is an amazing, com-major protein that cross-links to virtually all nascent plex, and efficient process. An E. coli cell utilizes up to chains of secretory and cytosolic proteins tested (Valent 20,000 ribosomes to produce an estimated total of et al., 1995; Hesterkamp et al., 1996). It may be posi-30,000 polypeptides per minute. Yet it is only the begin-tioned close to the exit site, as it cross-links to ribosome-ning. Each newly made protein must be folded into its associated chains 57 residues in length. TF, which has a correct tertiary structure. How this is achieved is one of central domain with homology to FK506 binding proteins the most basic, and complicated, questions of molecular (FKBPs), displays peptidyl-prolyl-cis-trans isomerase biology. and chaperone-like activities in vitro. It is unclear which When does the process of folding begin in the lifetime of these activities is required for its function in vivo. of a protein? Crystallographic data of bacterial ribo-Recent genetic studies provide clues suggesting a somes identified a peptide exit tunnel in the large subunit role of TF in protein folding in vivo. ⌬tig mutants lacking with a length of 100 A ˚ and an average diameter of about TF have no apparent phenotype and no major defects 20 A ˚ (Ban et al., 1999). This tunnel is long enough to in folding of newly synthesized proteins (Deuerling et accommodate extended chains of about 30 residues al., 1999; Teter et al., 1999). However, in the absence of and perhaps longer peptides in helical conformation, the Hsp70 chaperone DnaK, ⌬tig mutants are inviable. but certainly not peptides with tertiary structure. Once Upon depletion of DnaK in ⌬tig strains, more than 40 the N-terminal residues are synthesized, the nascent different newly synthesized cytosolic proteins aggre-polypeptide emerges into the crowded environment of gate, indicating a role for TF in folding of cytosolic pro-the cytosol. It is well established …