The amino acid sequence of a polypeptide defines both the folding pathway and the final three-dimensional structure of a protein. Eighteen amino acid substitutions have been identified in bacteriophage P22 coat protein that are defective in folding and cause their folding intermediates to be substrates for GroEL and GroES. These temperature-sensitive folding (tsf) substitutions identify amino acids that are critical for directing the folding of coat protein. Additional amino acid residues that are critical to the folding process of P22 coat protein were identified by isolating second site suppressors of the tsf coat proteins. Suppressor substitutions isolated from the phage carrying the tsf coat protein substitutions included global suppressors, which are substitutions capable of alleviating the folding defects of numerous tsf coat protein mutants. In addition, potential global and site-specific suppressors were isolated, as well as a group of same site amino acid substitutions that had a less severe phenotype than the tsf parent. The global suppressors were located at positions 163, 166, and 170 in the coat protein sequence and were 8 -190 amino acid residues away from the tsf parent. Although the folding of coat proteins with tsf amino acid substitutions was improved by the global suppressor substitutions, GroEL remained necessary for folding. Therefore, we believe that the global suppressor sites identify a region that is critical to the folding of coat protein.
Understanding the nature of protein grammar is critical because amino acid substitutions in some proteins cause misfolding and aggregation of the mutant protein resulting in a disease state. Amino acid substitutions in phage P22 coat protein, known as tsf (temperature-sensitive folding) mutations, cause folding defects that result in aggregation at high temperatures. We have isolated global su (suppressor) amino acid substitutions that alleviate the tsf phenotype in coat protein (Aramli, L. A., and Teschke, C. M. (1999) J. Biol. Chem. 274, 22217-22224). Unexpectedly, we found that a global su amino acid substitution in tsf coat proteins made aggregation worse and that the tsf phenotype was suppressed by increasing the rate of subunit assembly, thereby decreasing the concentration of aggregation-prone folding intermediates.The primary amino acid sequence of a polypeptide encodes all of the information necessary for folding and assembly pathways, as well as the native three-dimensional structure (2). Substitutions and deletions in the amino acid sequence of a protein can have a significant impact on the ability of a protein to fold or assemble properly. Depending on the protein, such changes in the amino acid sequence can lead to protein misfolding, mislocalization caused by misfolding, or aggregation (3, 4). Amino acid substitutions in p53 lead to a misfolding problem compromising the function of the protein, resulting in cancer (5). Further, there are the diseases that result from mislocalization caused by protein misfolding. For example, in ␣ 1 -antitrypsin deficiency, a single amino acid substitution results in the misfolding of ␣ 1 -antitrypsin, leading to the accumulation of long chain polymers within the hepatocyte. This leads to a reduction of the plasma concentrations of ␣-antitrypsin and predisposes individuals to emphysema and liver disease (6). Therefore, understanding the nature of protein grammar is of paramount interest.Although the process of protein folding is still not completely understood, it is known that larger proteins often have identifiable folding intermediates. These folding intermediates may interact inappropriately before reaching the native state. In fact, protein misfolding is a common problem faced by biotechnology companies that harvest proteins of commercial interest using recombinant DNA technologies in heterologous hosts (7-11). Often these proteins have problems with inclusion body formation, thereby decreasing the yield of pharmaceutically important products. Single amino acid substitutions can affect the folding pathway by shifting the folding from the productive pathway to off-pathway aggregation. For example, the amino acid substitutions in transthyretin causes a shift in the equilibrium between the native state and an aggregation-prone unfolding intermediate, resulting in amyloid formation. Individuals with any of the 50 known amino acid substitutions in transthyretin are predisposed to familial amyloidosis (12-15). During the folding of P22 tailspike proteins with tsf (te...
IgG fractions from subjects with SLE can be toxic to cultures of whole rat embryos in the absence of maternal tissues or influence. Such cultures of whole embryos may be useful to identify those antibodies that represent a risk for fetal loss as well as to understand their mechanisms of embryotoxicity.
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