The carboxy-terminal domain of heat-shock factor 1 is largely unfolded but can be induced to collapse into a compact, partially structured state

Heat-shock transcription factor 1 (HSF1) is a key regulator of the expression of heat-shock proteins during the heat-shock response. The C terminus of HSF1 (CT) contains both the regulatory and transcriptional activation domains. Predictors of natural disordered regions analysis predicts and our study demonstrates that CT is predominantly natively unfolded under physiological conditions but can be induced to fold into a number of structured states under different conditions. Under physiological conditions, CT exhibits a very low abundance of secondary and tertiary structures as observed by circular dichroism, no hydrophobic core as monitored by the 6-p-toluidino-2-naphthalenesulfonic acid (TNS)-binding assay, a large hydrodynamic radius as measured by size-exclusion chromatography-high-performance liquid chromatography, and high structural flexibility as probed by limited proteolysis. However, secondary-structure content significantly increases at high temperatures, in acidic pH, or in the presence of trimethylamine N-oxide, trifluoroethanol, or a cationic surfactant. Interestingly, the hydrophobicity of "folded" CT, as monitored by the TNS-binding assay, is enhanced by acidic pH and a cationic surfactant but not by trifluoroethanol. CT also displays different patterns in the proteolytic cleavage in acidic pH and in the presence of a cationic surfactant compared with that under native condition, suggesting that CT undergoes distinct structural rearrangements.