Arabidopsis MSBP1 is activated by HY5 and HYH and is involved in photomorphogenesis and brassinosteroid sensitivity regulation.

Membrane Steroid Binding Protein 1 (MSBP1) can bind steroids in vitro and negatively regulates brassinosteroid (BR) signaling, as well as cell elongation and expansion. Detailed analysis of the MSBP1 expression pattern based on quantitative real-time RT-PCR and promoter-GUS fusion studies revealed that MSBP1 expression in hypocotyls is stimulated by various light conditions. Interestingly, MSBP1 expression is greatly suppressed in hy5, hyh, or hy5 hyh mutants but enhanced in cop1 mutants. Further analysis employing a yeast one-hybrid assay, an electrophoretic mobility shift assay (EMSA), and a Chromatin IP (ChIP) assay confirmed the direct binding of Long Hypocotyl 5 (HY5) and HY5 Homolog (HYH) to the promoter region of MSBP1, indicating that MSBP1 is involved in light-regulated hypocotyl growth by serving as a direct target for HY5 and HYH. In addition, hy5 and hy5 hyh mutants show altered BR responses to light, which is consistent with the suppressed expression of MSBP1 in these mutants. These results suggest that light triggers MSBP1 expression through direct binding to and activation by HY5 and HYH, thereby inhibiting hypocotyl elongation. The findings also provide informative clues regarding the mechanisms for the negative regulation of BR sensitivity and photomorphogenesis during the dark-light transition.