Zebrafish HSF4: a novel protein that shares features of both HSF1 and HSF4 of mammals

Heat-shock proteins (hsps) have important roles in the development of the eye lens. We previously demonstrated that knockdown of hsp70 gene expression using morpholino antisense technology resulted in an altered lens phenotype in zebrafish embryos. A less severe phenotype was seen with knockdown of heat-shock factor 1 (HSF1), suggesting that, while it likely plays a role in hsp70 regulation during lens formation, other regulatory factors are also involved. Heat-shock factor 4 plays an important role in mammalian lens development, and an expressed sequence tag encoding zebrafish HSF4 has been identified. The deduced amino acid sequence shares structural similarities with mammalian HSF4 including the lack of an HR-C domain. However, the HR-C domain is absent due to a severe C-terminal truncation within zebrafish HSF4 (zHSF4) relative to the mammalian protein. Surprisingly, the amino acid composition of the zHSF4 DNA binding domain shares a greater degree of identity with HSF1 proteins than it does with mammalian HSF4 proteins. Consistent with this, the binding affinity of in vitro synthesized zHSF4 for discontinuous heat-shock response element sequences is more limited, similar to what has been previously observed for HSF1 proteins. Hsf4 mRNA is expressed in zebrafish adult eye tissue but is only observed in developing embryonic tissue at 60 h post-fertilization or later. This, together with the lack of an observable phenotype following morpholino-based antisense knockdown of hsf4, suggests that zHSF4 is unlikely to play a role in regulating early embryonic lens development.