Stress-70 proteins interactions with polypeptides

There is also evidence that the stress-70 proteins interact with extended, or unfolded, polypeptides. It has been demonstrated that a mitochondrial stress-70 protein is essential for import of proteins into mitochondria (Kang et al., 1990 Ostermann 1990). More specifically, polypeptides bind to, and can be cross-linked to, the mitochondrial stress-70 protein during import (Scherer et ai, 1990). It has further been demonstrated that cytoplasmic stress-70 proteins are essential for efficient transmembrane translocation of proteins into mitochondria or mi-crosomes (Chirico et al, 1988 Deshaies et al, 1988) this has led to the suggestion that the cytoplasmic stress-70 proteins may bind to and stabilize extended, unfolded conformations of polypeptide prior to their transmembrane translocation. 

The stress-70 proteins interact with a broad spectrum of polypeptide substrates, but they have some degree of specificity in their interactions. In several instances, it has been shown that a stress-70 protein can bind to proteins [e.g., bovine pancreatic trypsin inhibitor (BPTI), a-lactalbumin] that have been stabilized in a nonnative, or denatured, form by reduction and carboxymethylation of the cysteines that would normally form disulfides at the same time, they will not bind to the native forms of the same proteins (Liberek et al., 1991b Palleros et ai, 1991, 1992). This suggests that the peptide-binding activity of the stress-70 proteins discriminates in favor of polypeptides in a denatured, and possibly extended, conformation over those in a compact secondary and tertiary structure. NMR experiments demonstrating that the E. coli dnaK... 

Additionally, Welch and colleagues have demonstrated by immunopre-cipitation with monoclonal antibodies against cytoplasmic stress-70 proteins that these proteins appear to interact with a plethora of polypeptides in the cell, possibly recognizing them as nascent, unfolded polypeptides during translation (Beckmann et al., 1990). 

The denaturation of protein involves loss of their tertiary and secondary structures. This typically occurs by application of some external stresses, out of which thermal, interfacial and dehydration-related stresses are the most important stresses causing denaturation of proteins in drying processes. These stresses disrupt the tertiary structure, and subsequently the a-helix and j3-sheets of native proteins are turned into unfolded random shapes. When a protein is unfolded, the hydrophobically buried sites are exposed to the solvent and subsequently interact with interfaces and other unfolded polypeptides. The unfolded protein allows subsequent cross-linking interactions such as protein-protein hydrophobic, electrostatic, and disulfide-sulfhydryl interactions. These interactions result in aggregation, coagulation, and, finally, precipitation (Pelegrine and Gasparetto, 2005 Anandharamakrishnan et al, 2007). 

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