REPRESSION OF HUMAN HEAT-SHOCK FACTOR-1 ACTIVITY AT CONTROL TEMPERATURE BY PHOSPHORYLATION

Human heat shock transcription factor 1 (HSF1) is responsible for stre ss-induced transcription of heat shock protein genes. The activity of the HSF1 transcriptional activation domains is modulated by a separate regulatory domain, which confers repression at control temperature an d heat inducibility. We show here that two specific proline-directed s erine motifs are important for function of the regulatory domain: Muta tion of these serines to alanine derepresses HSF1 activity at control temperature, and mutation to glutamic acid, mimicking a phosphorylated serine, results in normal repression at control temperature and norma l heat shock inducibility. Tryptic mapping shows that these serines ar e the major phosphorylation sites of HSF1 at control temperature in vi vo. Stimulation of the Raf/ERK pathway in vivo results in an increased level of phosphorylation of these major sites and the regulatory doma in is an excellent substrate in vitro for the mitogen-activated MAPK/E RK. We conclude that phosphorylation of the regulatory domain of HSF1 decreases the activity of HSF1 at control temperature, and propose a m echanism for modification of HSF1 activity by growth control signals.