Clonal mosaic analysis of EMPTY PERICARP2 reveals nonredundant functions of the duplicated HEAT SHOCK FACTOR BINDING PROTEINs during maize shoot development

The paralogous maize proteins EMPTY PERICARP2 (EMP2) and HEAT SHOCK FACTOR BINDING PROTEIN2 (HSBP2) each contain a single recognizable motif: the coiled-coil domain. EMP2 and HSBP2 accumulate differentially during maize development and heat stress. Previous analyses revealed that EMP2 is required for regulation of heat shock protein (hsp) gene expression and also for embryo morphogenesis. Developmentally abnormal emp2 mutant embryos are aborted during early embryogenesis. To analyze EMP2 function during postembryonic stages, plants mosaic for sectors of emp2 mutant tissue were constructed. Clonal sectors of emp2 mutant tissue revealed multiple defects during maize vegetative shoot development, but these sector phenotypes are not correlated with aberrant hsp gene regulation. Furthermore, equivalent phenotypes are observed in emp2 sectored plants grown under heat stress and nonstress conditions. Thus, the function of EMP2 during regulation of the heat stress response can be separated from its role in plant development. The discovery of emp2 mutant phenotypes in postembryonic shoots reveals that the duplicate genes emp2 and hsbp2 encode nonredundant functions throughout maize development. Distinct developmental phenotypes correlated with the developmental timing, position, and tissue layer of emp2 mutant sectors, suggesting that EMP2 has evolved diverse developmental functions in the maize shoot.     

Gambogic acid and gambogenic acid induce a thiol-dependent heat shock response and disrupt the interaction between HSP90 and HSF1 or HSF2

Cancer cells rely on heat shock proteins (HSPs) for growth and survival. Especially HSP90 has multiple client proteins and plays a critical role in malignant transformation, and therefore different types of HSP90 inhibitors are being developed. The bioactive natural compound gambogic acid (GB) is a prenylated xanthone with antitumor activity, and it has been proposed to function as an HSP90 inhibitor. However, there are contradicting reports whether GB induces a heat shock response (HSR), which is cytoprotective for cancer cells and therefore a potentially problematic feature for an anticancer drug. In this study, we show that GB and a structurally related compound, called gambogenic acid (GBA), induce a robust HSR, in a thiol-dependent manner. Using heat shock factor 1 (HSF1) or HSF2 knockout cells, we show that the GB or GBA-induced HSR is HSF1-dependent. Intriguingly, using closed form ATP-bound HSP90 mutants that can be co-precipitated with HSF1, a known facilitator of cancer, we show that also endogenous HSF2 co-precipitates with HSP90. GB and GBA treatment disrupt the interaction between HSP90 and HSF1 and HSP90 and HSF2. Our study implies that these compounds should be used cautiously if developed for cancer therapies, since GB and its derivative GBA are strong inducers of the HSR, in multiple cell types, by involving the dissociation of a HSP90-HSF1/HSF2 complex.