No abstract

Cells subjected to a heat shock, or a variety of other stresses increase the synthesis of a set of proteins, known as heat shock proteins. This response is apparently universal, occurring in the entire range from bacterial to mammalian cells. In Escherichia coli heat shock protein synthesis transiently increases following a shift from 30 degrees C to 42 degrees C as a result of changes in transcription initiation at heat shock promoters. Heat shock promoters are recognized by RNA polymerase containing a sigma factor of relative molecular mass (Mr) 32,000 (32K) E sigma 32 and not E sigma 70, the major form of RNA polymerase holoenzyme. To determine whether changes in the concentration of sigma 32 regulate this response, we measured the amount of sigma 32 before and after shift to high temperature and found that it increased transiently during heat shock as a result of changes in sigma 32 synthesis and stability. Our results indicate that sigma 32 is directly responsible for regulation of the heat shock response.

CTLA-4, a negative regulator of T cell function, was found to associate with the T cell receptor (TCR) complex ζ chain in primary T cells. The association of TCRζ with CTLA-4, reconstituted in 293 transfectants, was enhanced by p56 lck -induced tyrosine phosphorylation. Coexpression of the CTLA-4–associated tyrosine phosphatase, SHP-2, resulted in dephosphorylation of TCRζ bound to CTLA-4 and abolished the p56 lck -inducible TCRζ–CTLA-4 interaction. Thus, CTLA-4 inhibits TCR signal transduction by binding to TCRζ and inhibiting tyrosine phosphorylation after T cell activation. These findings have broad implications for the negative regulation of T cell function and T cell tolerance.

The Escherichia coli DnaK heat shock protein has been identified previously as a negative regulator of E. coli heat shock gene expression. We report that two other heat shock proteins, DnaJ and GrpE, are also involved in the negative regulation of heat shock gene expression. Strains carrying defective dnaK, dnaj, or grpE alleles have enhanced synthesis of heat shock proteins at low temperature and fail to shut off the heat shock response after shift to high temperature. These regulatory defects are due to the loss of normal control over the synthesis and stability of CT^^, the alternate RNA polymerase tr-factor required for heat shock gene expression. We conclude that DnaK, DnaJ, and GrpE regulate the concentration of o-^^. We suggest that the synthesis of heat shock proteins is controlled by a homeostatic mechanism linking the function of heat shock proteins to the concentration of a^^.