The scaffold proteins of lignin biosynthetic cytochrome P450 enzymes

First author: Mingyue Gou; Affiliations: Brookhaven National Laboratory (“布鲁克海文”国立实验室): Upton, USA

Corresponding author: Chang-Jun Liu

Lignin is a complex and irregular biopolymer (不规则生物聚合物) of crosslinked (交联的) phenylpropanoid (苯丙素) units in plant secondary cell walls. Its biosynthesis requires three endoplasmic reticulum (ER; 内质网)-resident cytochrome P450 monooxygenases (单加氧酶), C4H, C3ʹH and F5H, to establish the structural characteristics of its monomeric precursors. These P450 enzymes were reported to associate with each other or potentially with other soluble monolignol biosynthetic enzymes to form an enzyme complex or a metabolon (代谢区室). However, the molecular basis governing such enzyme or pathway organization remains elusive. Here, we show that Arabidopsis membrane steroid-binding proteins (MSBPs; 膜甾体结合蛋白) serve as a scaffold to physically organize monolignol P450 monooxygenases, thereby regulating the lignin biosynthetic process. We find that although C4H, C3ʹH and F5H are in spatial proximity to each other on the ER membrane in vivo, they do not appear to directly interact with each other. Instead, two MSBP proteins physically interact with all three P450 enzymes and, moreover, MSBPs themselves associate as homomers (同型异构体) and heteromers (异型异构体) on the ER membrane, thereby organizing P450 clusters. Downregulation of MSBP genes does not affect the transcription levels of monolignol biosynthetic P450 genes but substantially impairs the stability and activity of the MSBP-interacting P450 enzymes and, consequently, lignin deposition, and the accumulation of soluble phenolics in the monolignol branch but not in the flavonoid pathway. Our study suggests that MSBP proteins are essential structural components in the ER membrane that physically organize and stabilize the monolignol biosynthetic P450 enzyme complex, thereby specifically controlling phenylpropanoid-monolignol branch biosynthesis.

木质素是植物次生细胞壁中由交联的苯丙素单元构成的不规则生物聚合物。其生物合成需要三个定位于内质网上的细胞色素P450单加氧酶，分别是C4H、C3ʹH和F5H，这三个酶能够建立起木质素单体前体的结构特性。之前的报道显示这三个P450酶相互关联或者与其它的一些可溶性木质素单体生物合成酶形成酶复合物或者是代谢区室（注：代谢通路中某些酶形成的疏松的复合体三维结构）。然而目前，我们对于这些酶是如何发挥作用的分子机制还了解得很少。本文报道了拟南芥膜甾体结合蛋白MSBP作为物理支架来协调木质素单体P450单加氧酶，进而作用于木质素生物合成进程。作者发现尽管体内环境下内质网膜上C4H、C3ʹH和F5H相互之间在空间上比较接近，然而它们之间并不发生直接的相互作用。相反，两个MSBP蛋白物理上与这三个P450单加氧酶能够相互作用，并且MSBPs本身在内质网膜上相互交联形成同型异构体和异型异构体，因此能够协调一大簇的P450酶。下调MSBP基因并不影响木质素单体生物合成P450基因的转录水平，但确实会损害与MSBP相互作用的P450酶的稳定性和活性，并最终导致木质素沉积和可溶性酚醛树脂在木质素单体分支而非类黄酮通路上积累。本文的研究表明MSBP蛋白对于内质网膜上的结构组分至关重要，其在物理上组织和稳定木质素单体生物合成的P450酶复合物，因此特异控制苯丙素-木质素单体分支的生物合成。

通讯：Chang-Jun Liu(https://www.bnl.gov/biosciences/staff/Liu.php)

个人简介：1999年，中科院上海植物生理研究所，博士。

研究方向：苯丙烷-木质素生物合成以及相关调控机制。

doi: 10.1038/s41477-018-0142-9

Journal: Nature Plants

Published online: 30 April, 2018

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