Multifaceted roles of Arabidopsis heat shock factor binding protein in plant growth, development, and heat shock response

Heat shock factor-binding protein (HSBP) is a 10 kDa protein in plants and animals and consists exclusively of a coiled-coil domain. During the recovery phase following a heat shock response, HSBP relocates from the cytoplasm to the nucleus in Arabidopsis (Arabidopsis thaliana), where it interacts with heat shock factors (HSFs). Here, we found that 16 out of the 19 functional known HSFs can interact with HSBP. Besides, our results indicate that HSBP negatively regulates HSF gene expression during normal growth conditions and recovery from heat shock. Expanding our understanding of HSBP's physiological functions during regular growth, co-immunoprecipitation and mass spectrometry analysis identified 16 coiled-coil domain-containing proteins co-immunoprecipitated with HSBP. These proteins encompass HSP70s, all components of the MAIGO2 complex, COP1-interactive protein1 (CIP1), CIP1-like protein, kinesin-like protein for actin-based chloroplast movement1 and 2 (KAC1/2), and HSBP itself. By examining mutant plants lacking HSBP and its interacting proteins, we elucidated their functional relationships. Our findings underscore the indispensability of the HSBP coiled-coil heptad repeat for interacting with its partners and its crucial role in growth, development, and the heat shock response. In addition to its involvement in heat shock response, Arabidopsis HSBP, a discreet small regulatory protein, exerts multiple regulatory functions in hypocotyl elongation, flowering time, chloroplast photo-relocation, and seed development.