ACAD9 belongs to the acyl-CoA dehydrogenase family, which catalyzes the α-β dehydrogenation of fatty acyl-CoA thioesters. Thus, it is involved in fatty acid β-oxidation (FAO). However, it is now known that the primary function of ACAD9 is as an essential chaperone for mitochondrial respiratory complex 1 assembly. ACAD9 interacts with ECSIT and NDUFAF1, forming the mitochondrial complex 1 assembly (MCIA) complex. Although the role of MCIA in the complex 1 assembly pathway is well studied, little is known about the molecular mechanism of the interactions among these three assembly factors. Our current studies reveal that when ECSIT interacts with ACAD9, the flavoenzyme loses the FAD cofactor and consequently loses its FAO activity, demonstrating that the two roles of ACAD9 are not compatible. ACAD9 binds to the carboxy-terminal half (C-ECSIT), and NDUFAF1 binds to the amino-terminal half of ECSIT. Although the binary complex of ACAD9 with ECSIT or with C-ECSIT is unstable and aggregates easily, the ternary complex of ACAD9-ECSIT-NDUFAF1 (i.e., the MCIA complex) is soluble and extremely stable. Molecular modeling and SAXS studies of the MCIA complex identified the possible interaction sites between the three assembly factors and binding sites for other assembly factors, including complex 1 subunits. Furthermore, we have mapped over 40 currently known pathogenic mutation sites onto the homology-modeled ACAD9 structure, giving us the structural basis for their involvement in diseases that result from complex 1 deficiency.

中文翻译：

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线粒体呼吸复合物1装配中ACAD9的分子机制

ACAD9属于酰基辅酶A脱氢酶家族，可催化​​脂肪酰基辅酶A硫酯的α-β脱氢。因此，它参与了脂肪酸β-氧化（FAO）。但是，现在知道，ACAD9的主要功能是作为线粒体呼吸复合物1装配的必需伴侣。ACAD9与ECSIT和NDUFAF1相互作用，形成线粒体复合体1装配体（MCIA）复合体。尽管已经很好地研究了MCIA在复杂的1组装途径中的作用，但对这三个组装因子之间相互作用的分子机理了解甚少。我们目前的研究表明，当ECSIT与ACAD9相互作用时，黄素酶会丢失FAD辅因子，从而失去其FAO活性，这表明ACAD9的两个作用不兼容。ACAD9结合到羧基末端的一半（C-ECSIT），NDUFAF1结合ECSIT的氨基末端一半。尽管ACAD9与ECSIT或C-ECSIT的二元复合物不稳定且易于聚集，但ACAD9-ECSIT-NDUFAF1的三元复合物（即MCIA复合物）可溶且极其稳定。MCIA复合物的分子建模和SAXS研究确定了三个装配因子与其他装配因子（包括复合物1亚基）的结合位点之间可能的相互作用位点。此外，我们已经在同源模型ACAD9结构上绘制了40多个当前已知的致病突变位点，从而为它们参与复杂1缺陷导致的疾病提供了结构基础。ACAD9-ECSIT-NDUFAF1的三元复合物（即MCIA复合物）可溶且非常稳定。MCIA复合物的分子建模和SAXS研究确定了三个装配因子与其他装配因子（包括复合物1亚基）的结合位点之间可能的相互作用位点。此外，我们已经在同源模型ACAD9结构上绘制了40多个当前已知的致病突变位点，从而为它们参与复杂1缺陷导致的疾病提供了结构基础。ACAD9-ECSIT-NDUFAF1的三元复合物（即MCIA复合物）可溶且非常稳定。MCIA复合物的分子建模和SAXS研究确定了三个装配因子与其他装配因子（包括复合物1亚基）的结合位点之间可能的相互作用位点。此外，我们已经在同源模型ACAD9结构上绘制了40多个当前已知的致病突变位点，从而为它们参与复杂1缺陷导致的疾病提供了结构基础。