Formation of nuclear HSF1 granules varies depending on stress stimuli.

In concert with the stress-induced activation of human heat shock factor 1 (HSF1), the factor becomes inducibly phosphorylated and accumulates into nuclear granules. To date, these processes are not fully understood. Here, we show that although stress caused by the proteasome inhibitors MG132 and clasto-lactacystine beta-lactone induces the expression of Hsp70, the formation of HSF1 granules is affected differently in comparison to heat shock. Furthermore, proteasome inhibition increases serine phosphorylation on HSF1, but to a lesser extent than heat stress. Our results suggest that, depending on the type of stress stimulus, the multiple events associated with HSF1 activation might be affected differently.