Characterization of denatured protein inducers of the heat shock (stress) response in Xenopus laevis oocytes.
In addition to thermal stress, a large variety of physical and chemical treatments are known to induce heat shock gene expression. Denatured protein, thought to result from the stress condition, has been postulated to act as the common signal. Accordingly, of three pairs of native and denatured proteins injected into Xenopus laevis oocytes, only the denatured derivatives induced expression of a reporter gene from a heat shock promoter (Ananthan, J., Goldberg, A. L., and Voellmy, R. (1986) Science 232, 522-525). These observations are extended here. Protein denaturation per se is shown to be insufficient for heat shock induction; although reduced and carboxymethylated bovine serum albumin (rcm-BSA) and alpha-crystallin elicited a stress response, many other denatured proteins had no effect. Methylation of protein lysines, done to prevent ubiquitination, suppressed heat shock induction by rcm-BSA, but enhanced induction by alpha-crystallin. Thus, the potential for a protein to be ubiquitinated is independent of its ability to induce the stress response. Instead, aggregation distinguished the proteins that were effective stress inducers, and the formation of large aggregates correlated with the magnitude of the response. This correlation may derive in part from decreased in vivo degradation rates of the inducer proteins. An apparent requirement for stress response induction that the inducer proteins be injected directly into the oocyte nucleus may relate to this issue of in vivo stability. The dependence of the stress response on the amount of injected protein is non-linear and of a form consistent with the titration of a factor that otherwise suppresses heat shock gene expression.