Regulated expression of heat shock factor 1 isoforms with distinct leucine zipper arrays via tissue-dependent alternative splicing.

HSF1 mediates the stress induced expression of heat shock proteins, referred to as the cellular stress response. Previous results indicated that mammalian cells express two distinct HSF1 protein isoforms, with molecular sizes of 69 kDa (HSF1-beta) and 71 kDa (HSF1-alpha). The purpose of this study was to determine the mechanism by which these two HSF1 protein isoforms are generated. Our results show that mammalian cells express two distinct HSF1 mRNA isoforms which arise via alternative splicing of the HSF1 pre-mRNA. The two HSF1 mRNA isoforms differ by a single 66 bp exon of the HSF1 gene which is spliced into the HSF1-alpha mRNA isoform but skipped in the HSF1-beta mRNA isoform. This 66 bp exon encodes a 22 amino acid sequence, whose molecular weight (2.3 kDa) matches the difference in size between the HSF1-beta and HSF1-alpha protein isoforms (69 and 71 kDa). Further analysis reveals that this extra 22 amino acid sequence, whose insertion site in the HSF1-alpha isoform is located immediately adjacent to a C-terminal leucine zipper motif (leucine zipper 4) previously shown to be involved in maintenance of HSF1 in the non-DNA-binding control form, contains an additional, previously unidentified leucine zipper motif (leucine zipper 5). Our results also show that the levels of the two HSF1 isoforms are regulated in a tissue dependent manner, with testis expressing higher levels of the HSF1-beta isoform while heart and brain express higher levels of the HSF1-alpha isoform. These results demonstrate a new mechanism by which HSF1 expression is regulated in mammalian cells and suggest a potential role for the HSF1 isoforms in mediating tissue-dependent regulation of the cellular stress response.