Willows R D, Hansson A, Birch D, Al-Karadaghi S, Hansson M
Department of Biological Science, Macquarie University, Macquarie Drive, North Ryde 2109, Australia.
J Struct Biol. 2004 Apr-May;146(1-2):227-33. doi: 10.1016/j.jsb.2003.11.019.
BchI, belonging to the AAA+ -protein family, forms the enzyme magnesium chelatase together with BchD and BchH. This enzyme catalyses the insertion of Mg2+ into protoporphyrin IX upon ATP hydrolysis. Previous studies have indicated that BchI forms ATP-dependent complexes and it is a member of the AAA+ -protein family (ATPases associated with various cellular activities) and it was suggested based on structural homology that the BchI formed hexameric complexes. AAA+ -proteins are Mg2+ -dependent ATPases that normally form oligomeric ring complexes in the presence of ATP. Single particle analysis of fully formed ring complexes of BchI observed by negative staining EM indicate that the BchI has strong 6- and 2-fold rotational symmetries and a weaker 4-fold rotational symmetry which are reminiscent of DNA helicase. A 2D average of the fully formed BchI-ATP ring complex is presented here from images of the complex obtained from negative staining EM. Other complexes are also observed in the EM micrographs and the class averages of these are indicative of the fragility and dynamic nature of the BchI complex which has been reported and they are suggestive of partially circular complexes with six or less protomers per particle. The resolution of the average circular complex is estimated at approximately 30A and it is similar in shape and size to an atomic resolution hexameric model of BchI rendered at 30A.
BchI属于AAA +蛋白家族，与BchD和BchH一起构成镁螯合酶。该酶在ATP水解时催化Mg2 +插入原卟啉IX中。先前的研究表明，BchI形成ATP依赖性复合物，它是AAA +蛋白家族（与各种细胞活动相关的ATP酶）的成员，并且基于结构同源性表明BchI形成六聚体复合物。AAA +蛋白是Mg2 +依赖性ATP酶，通常在ATP存在下形成寡聚环复合物。通过负染色电子显微镜观察到的BchI完全形成的环复合物的单颗粒分析表明，BchI具有很强的6倍和2倍旋转对称性以及较弱的4倍旋转对称性，这让人联想到DNA解旋酶。这里从负染色电子显微镜获得的复合物图像展示了完全形成的BchI-ATP环复合物的二维平均值。在电子显微镜照片中也观察到其他复合物，这些复合物的类平均值表明了已报道的BchI复合物的脆弱性和动态性质，并且暗示了每个颗粒具有六个或更少原体的部分圆形复合物。平均圆形复合物的分辨率估计约为30埃，其形状和大小与在30埃下呈现的BchI原子分辨率六聚体模型相似。
