Developmental regulation of Hsp32, a small heat shock protein in Dictyostelium discoideum.

We have examined the expression and heat inducibility of Hsp32, a novel small heat shock protein in Dictyostelium discoideum. Both Hsp32 and its mRNA are abundant in amoebae growing at physiological temperatures. Levels of Hsp32 remain high during the initial phases of development, including the formation of tipped mounds. After that stage, Hsp32 levels decrease, reaching barely detectable levels in culminating cells. In contrast, most of the hsp32 mRNA is rapidly degraded within the first few hours of starvation-induced development. Cells retain a new low steady state level of the mRNA throughout the rest of the developmental cycle. However, when cells undergo dedifferentiation, they reaccumulate high levels of hsp32 mRNA just prior to cell division. The heat inducibility of Hsp32 and its mRNA is maximal in growing cells and decreases as cells progress in their developmental program. The data suggest that Hsp32 is associated with a growth and/or survival function that is gradually eliminated during development.