There is no consensus on whether inactive HSF1 monomers localize in the nucleus or in the cytosol (Sarge KD et al. 1993; Zuo J et al. 1995; Mercier PA et al. 1999; Vujanac M et al. 2005). Moreover, inactive HSF1 was reported to constitutively shuttle between the nucleus and the cytoplasm in mammalian cells (Vujanac M et al. 2005). However, active HSF1 trimers were shown to rapidly accumulate in the nucleus where they bind to heat shock elements (HSE) present within promoters of hsp genes (Wang Y and Morgan WD 1994; Herbomel G et al. 2013).
Heat shock response in human and monkey cells (but not rodent cells) is also associated with the stress-induced relocalization of HSF1 within the nucleus not only on hsp gene promoters but also into specific subnuclear organelles termed nuclear stress bodies (nSBs, also known as HSF1 granules) (Sarge KD et a. 1993; Cotto JJ et al. 1997; Jolly C et al. 1999). nSBs are rarely detectable in unstressed cells but their number drastically increases after heat shock. Formation of nSBs is initiated by the interaction between HSF1 and pericentric tandem repeats of satellite III sequences on chromosome 9, where sat III repeats are transcribed by RNA polymerase II in an HSF1-dependent manner. (Jolly C et al. 2002, 2004). HSF1 can also bind to DNA regions enriched in sat II and sat III repeated sequences detected on other human chromosomes (Eymery A et al. 2010). The functional relevance of HSF1 granules and their transcripts remains an open question.