Phosphorylation of serine 230 promotes inducible transcriptional activity of heat shock factor 1

Heat shock factor 1 (HSF1) is a serine-rich constitutively phosphorylated m ediator of the stress response. Upon stress, HSF1 forms DNA-binding trimers , relocalizes to nuclear granules, undergoes inducible phosphorylation and acquires the properties of a transactivator. HSF1 is phosphorylated on mult iple sites, but the sites and their function have remained an enigma. Here, we have analyzed sites of endogenous phosphorylation on human HSF1 and dev eloped a phosphopeptide antibody to identify Ser230 as a novel in vivo phos phorylation site. Ser230 is located in the regulatory domain of HSF1, and p romotes the magnitude of the inducible transcriptional activity. Ser230 lie s within a consensus site for calcium/calmodulin-dependent protein kinase I I (CaMKII), and CaMKII overexpression enhances both the level of in vivo Se r230 phosphorylation and transactivation of HSF1. The importance of Ser230 was further established by the S230A HSF1 mutant showing markedly reduced a ctivity relative to wild-type HSF1 when expressed in hsf1(-/-) cells. Our s tudy provides the first evidence that phosphorylation is essential for the transcriptional activity of HSF1, and hence for induction of the heat shock response.