Final answer:
HSF1 is inactive and monomeric when unstressed, bound to heat shock proteins in the cytoplasm. Upon stress, it trimerizes, translocates to the nucleus, and activates heat shock gene transcription.
Step-by-step explanation:
When HSF1 is unstressed, it exists in a stable, inactive form in the cytoplasm. In this state, HSF1 is generally found as a monomer that is complexed with heat shock proteins (HSPs), which prevent it from entering the nucleus and initiating the heat shock response. Upon stress, such as elevated temperatures, HSF1 is released from these inhibitory complexes and undergoes trimerization.
This structural change allows HSF1 to translocate to the nucleus, where it binds to heat shock elements (HSEs) in the DNA and activates the transcription of heat shock genes, leading to the production of more HSPs. Hence, the active and inactive states of HSF1 are regulated through its association with HSPs and through post-translational modifications that either promote or inhibit its DNA-binding activity when cells are under stress or in a normal physiological state, respectively.