Structural Insights into the MMACHC-MMADHC Protein Complex Involved in Vitamin B12 Trafficking
Structural Insights into the MMACHC-MMADHC Protein Complex Involved in Vitamin B12 Trafficking
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DOI：
10.1074/jbc.m115.683268
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发表时间：
2015-12-04
影响因子：
4.8
通讯作者：
Yue, Wyatt W.
中科院分区：
文献类型：
--
作者：
Froese, D. Sean;Kopec, Jolanta;Yue, Wyatt W.
维生素B-12（钴胺素，Cbl）转化为辅因子形式甲基-Cbl（MeCbl）和腺苷-Cbl（Adenosyl-Cbl）是两种关键酶（线粒体甲基丙二酰-CoA还原酶和细胞溶质甲硫氨酸合酶）功能所必需的。细胞内蛋白MMAHC和MMADHC在处理和靶向Cbl辅因子到其目的地酶中发挥重要作用，最近的证据表明，它们可能在执行这些必要的运输功能时相互作用。为了更好地理解这种相互作用的分子基础，我们绘制了所需的关键蛋白质区域，表明Cbl可能在与MMADHC相互作用之前被MMAHC处理，并通过不同的机制鉴定了两种蛋白质上干扰复合物形成的患者突变。我们进一步报告了MMADHC C-末端区域在2.2埃分辨率下的晶体结构，揭示了与MMAHC具有惊人同源性的修饰的硝基还原酶折叠，尽管它们的序列保守性很差。由于MMADHC-没有表现出已知的酶活性，我们建议它作为第一个已知的蛋白质重新利用硝基还原酶折叠仅用于蛋白质-蛋白质相互作用。使用小角X-射线散射，我们揭示了MMAHC-MMADHC复合物作为1：1异二聚体，并提供了这种相互作用的结构模型，其中相互作用区域与MMAHC-Cbl结合位点重叠。总之，我们的研究结果提供了新的结构证据和机制洞察到一个基本的生物过程，即细胞内的“贩运分子伴侣”高度特异性的微量元素辅因子功能通过蛋白质-蛋白质相互作用，这是破坏遗传性疾病突变。
Conversion of vitamin B-12 (cobalamin, Cbl) into the cofactor forms methyl-Cbl (MeCbl) and adenosyl-Cbl (AdoCbl) is required for the function of two crucial enzymes, mitochondrial methylmalonyl-CoA mutase and cytosolic methionine synthase, respectively. The intracellular proteins MMACHC and MMADHC play important roles in processing and targeting the Cbl cofactor to its destination enzymes, and recent evidence suggests that they may interact while performing these essential trafficking functions. To better understand the molecular basis of this interaction, we have mapped the crucial protein regions required, indicate that Cbl is likely processed by MMACHC prior to interaction with MMADHC, and identify patient mutations on both proteins that interfere with complex formation, via different mechanisms. We further report the crystal structure of the MMADHC C-terminal region at 2.2 angstrom resolution, revealing a modified nitroreductase fold with surprising homology to MMACHC despite their poor sequence conservation. Because MMADHC-demonstrates no known enzymatic activity, we propose it as the first protein known to repurpose the nitroreductase fold solely for protein-protein interaction. Using small angle x-ray scattering, we reveal the MMACHC-MMADHC complex as a 1: 1 heterodimer and provide a structural model of this interaction, where the interaction region overlaps with the MMACHC-Cbl binding site. Together, our findings provide novel structural evidence and mechanistic insight into an essential biological process, whereby an intracellular "trafficking chaperone" highly specific for a trace element cofactor functions via protein-protein interaction, which is disrupted by inherited disease mutations.