National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 300 Fenglin Road, 200032 Shanghai, PR China.
Mol Plant. 2011 Nov;4(6):1092-104. doi: 10.1093/mp/ssr049. Epub 2011 Jun 29.
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.
膜甾体结合蛋白 1（MSBP1）可以在体外结合甾体，并负调控油菜素内酯（BR）信号转导以及细胞伸长和扩展。基于定量实时 RT-PCR 和启动子-GUS 融合研究对 MSBP1 表达模式的详细分析表明，下胚轴中的 MSBP1 表达受各种光照条件的刺激。有趣的是，在 hy5、hyh 或 hy5 hyh 突变体中，MSBP1 的表达受到极大抑制，但在 cop1 突变体中增强。进一步的分析利用酵母单杂交分析、电泳迁移率变动分析（EMSA）和染色质免疫沉淀（ChIP）分析证实了长下胚轴 5（HY5）和 HY5 同源物（HYH）与 MSBP1 启动子区域的直接结合，表明 MSBP1 通过作为 HY5 和 HYH 的直接靶标参与光调控下胚轴生长。此外，hy5 和 hy5 hyh 突变体表现出对光的 BR 反应改变，这与这些突变体中 MSBP1 的表达受抑制一致。这些结果表明，光通过直接结合和激活 HY5 和 HYH 触发 MSBP1 表达，从而抑制下胚轴伸长。这些发现还为黑暗-光照过渡期间 BR 敏感性和光形态发生的负调控机制提供了有价值的线索。
