SPECIFIC INTRANUCLEOLAR DISTRIBUTION OF HSP70 DURING HEAT-SHOCK IN POLYTENE CELLS

Hsp70, the most abundant and conserved heat shock protein, has been de scribed as strongly concentrating in the nucleolus during heat shock, The important metabolic processes that take place in the nucleolus, rD NA transcription, processing, and assembling with ribosomal proteins, and the nucleolar architecture itself are very sensitive to temperatur e changes. In this work, we have analyzed in detail the nucleolar chan ges, in structure and activity, induced by temperature in Chironomus t hummi salivary gland cells and the fine subnucleolar localization of H sp70 during heat shock, The optimum temperature chosen to induce the h eat shock response was 35 degrees C. Under these conditions transcript ion of heat shock genes, inactivation of previously active genes and m aximum synthesis of Hsps take place, while survival of larvae and reco very were ensured. After 1 h at 35 degrees C, nucleoli change from a u niform control pattern to a segregated pattern of nucleolar components that can be observed even at the light microscopic level, The dense f ibrillar component (DFC) and the granular component appeared perfectly differentiated and spatially separated, the former occupying mainly t he central inner region surrounded by a rim of granular component. Hsp 70 was specifically localized within the DFC upon heat shock as shown by immunolocalization by both light and electron miccroscopy. Pulse la beling with [H-3]uridine proves that rRNA transcription continues duri ng heat shock. The pattern of Hsp70 distribution within the nucleolus correlates with that of newly produced rRNA transcripts. Hsp70 also co localizes with RNA polymerase I, both being restricted to the DFC. The se data show that the DFC seems to be the intranucleolar target for Hs p70 in heat-shocked cells. We discuss these results in relation to the possible function of Hsp70 in the first steps of preribosome synthesi s. (C) 1997 Academic Press.