Genetic and biochemical analyses have revealed the presence in E. coli of two distinct sets of genes that respond to the accumulation of misfolded or damaged polypeptide chains resulting from a heat shock. The classical heat shock response is triggered by protein misfolding in the cytoplasm and is mediated by the RNA polymerase minor sigma factor, sigma32 (the product of rpoH). A second heat shock response, mediated by a second minor sigma factor, sigmaE (the product of rpoE), is triggered by the accumulation of misfolded proteins in the extracytoplasmic space (ie., the periplasm and outer membrane compartments of E. coli). This proposal investigates two major questions: (1) How are misfolded proteins detected in the periplasm? (2) How does detection of misfolded proteins trigger sigmaE-dependent transcription of specific heat shock genes, including rpoH)? These questions will be addressed by examining the roles of four proteins, sigmaE, RseA, RseB, and RseC. Genes encoding the last three of these proteins were identified by mutational studies conducted by the PI, which permitted the development of a model in which contact with misfolded proteins in the periplasm triggers a conformational change in the transmembrane protein RseA, relaxing its inhibitory effect on cytoplasmic sigmaE. These interactions are modulated positively and negatively by RseB and RseC. The proposal is designed to use a combination of genetic and biochemical experiments to elucidate the precise function of these proteins. The goals are to determine whether RseA or RseB is the actual sensor of misfolded proteins and to determine how the signal is relayed to effect the release of sigmaE from RseA. The role of sigmaE-dependent operons in the physiological response to the accumulation of misfolded polypeptide will also be investigated.
