Developmental effects of ectopic expression of the glucocorticoid receptorDNA binding domain are alleviated by an amino acid substitution that interferes with homeodomain binding

Steroid hormone receptors are distinguished from other members of the nucle ar hormone receptor family through their association with heat shock protei ns and immunophilins in the absence of ligands. Heat shock protein associat ion represses steroid receptor DNA binding and protein-protein interactions with other transcription factors and facilitates hormone binding. In this study, we investigated the hormone-dependent interaction between the DNA bi nding domain (DBD) of the glucocorticoid receptor (GR) and the POU domains of octamer transcription factors 1 and 2 (Oct-1 and Oct-2, respectively), O ur results indicate that the GR DBD binds directly, not only to the homeodo mains of Oct-1 and Oct-2 but also to the homeodomains of several other home odomain proteins. As these results suggest that the determinants for bindin g to the GR DBD are conserved within the homeodomain, we examined whether t he ectopic expression of GR DBD peptides affected early embryonic developme nt. The expression of GR DBD peptides in one-cell-stage zebra fish embryos severely affected their development, beginning with a delay in the epibolic movement during the blastula stage and followed by defects in convergenee- extension movements during gastrulation, as revealed by the abnormal patter ns of expression of several dorsal gene markers, In contrast, embryos injec ted with mRNA encoding a GR peptide with a point mutation that disrupted ho meodomain binding or with mRNA encoding the DBD of the closely related mine ralocorticoid receptor, which does not bind octamer factors, developed norm ally. Moreover, coinjection of mRNA encoding the homeodomain of Oct-2 compl etely rescued embryos from the effects of the GR DBD, These results highlig ht the potential of DNA-independent effects of GR in a whole-animal model a nd suggest that at least some of these effects may result from direct inter actions with homeodomain proteins.