Small heat shock proteins of Drosophila associate with the cytoskeleton

Fractionation of heat-shocked Drosophila melanogaster Kc cells reveals that both the small heat shock proteins (hsp28, -26, -23, and -22) and vimentin-like intermediate filament proteins (IFPs) are abundantly represented in the nuclear fraction. Cofractionation of the IFPs with nuclei is due to the collapse of the IFP network against the nucleus upon heat shock, raising the possibility that cofractionation of the small hsps is by a similar mechanism. Indirect immunofluorescence supports this possibility. In salivary glands, both the hsps and the IFPs are cytoplasmic after mild-to-moderate heat shocks and only enter the nucleus upon severe--indeed, lethal--shocks. Double-label experiments with Schneider line 2 cells show that the IFPs and small hsps colocalize to the same perinuclear aggregates in 70% of the cells examined. Thus, the small hsps are associated with the cytoskeleton rather than with nuclear structures.