The developmental expression of the heat-shock response in Xenopus laevis

We have investigated the Xenopus heat-shock response of somatic cells, oocytes and embryos. Xenopus defolliculated oocytes displayed a highly variable response to heat shock depending on the culture medium. Intact follicles, however, respond to heat stress by synthesizing an invariant pattern of hsps. Although a subset of the hsp70/68 complex is expressed constitutively in the absence of heat shock in oocytes and embryos (hsc70), actual induction of hsps in response to stress does not occur until the blastula stage when transcription of the zygotic genome is first activated. By gastrulation, the hsps of somatic cells, including members of the hsp30/26 complex, were expressed coordinately in response to heat shock. We further show that Xenopus hsps have different solubilities perhaps reflecting their different subcellular locations. The 26,000-30,000 Mr complex (hsp30/26) was present almost exclusively in a detergent-insoluble fraction, as was 25-50% of the hsp70/68 complex and greater than 50% of hsp56, suggesting that these hsps may be associated with the cytoskeleton during a heat shock. In contrast, the other Xenopus hsps (hsp86, hsp75 and hsp61) were totally solubilized in a low-salt buffer.