Heterohexamers Formed by CcmK3 and CcmK4 Increase the Complexity of Beta Carboxysome Shells.
Heterohexamers Formed by CcmK3 and CcmK4 Increase the Complexity of Beta Carboxysome Shells.
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CcmK3 和 CcmK4 形成的异六聚体增加了 Beta 羧基体壳的复杂性。
作者：
Sommer,Manuel;Sutter,Markus;Gupta,Sayan;Kirst,Henning;Turmo,Aiko;Lechno-Yossef,Sigal;Burton,RodneyL;Saechao,Christine;Sloan,NancyB;Cheng,Xiaolin;Chan,Leanne-JadeG;Petzold,ChristopherJ;Fuentes-Cabrera,Miguel;Ralston,CorieY;
细菌微区室（BMC）将酶封装在选择性渗透的蛋白质外壳内。羧化体是含有核酮糖-1，5-二磷酸羧化酶、加氧酶和碳酸酐酶的BMC，其增强二氧化碳固定。羧基体外壳由三种结构特征的蛋白质类型组成，每种蛋白质类型都以它们形成的寡聚体命名：BMC-H（六聚体），BMC-P（五聚体）和BMC-T（三聚体）。这三种蛋白质类型形成环状同源寡聚体，在对称中心具有孔，使代谢物能够穿过外壳运输。羧基体壳含有多个BMC-H旁系同源物，每个旁系同源物在孔周围具有明显保守的残基，这些残基被认为与特定的代谢物相关。我们研究了BMC-H基因ccmK 3和ccmK 4对β-羧基体壳组成的调控。我们在细长聚球藻PCC 7942中制作了ccmK 3和ccmK 4的单缺失和双缺失突变体，并表明与CcmK 3不同，CcmK 4是最佳生长所必需的。与其他CcmK蛋白相比，CcmK 3不形成同六聚体;相反，CcmK 3与CcmK 4以1：2的化学计量形成异六聚体。CcmK 3-CcmK 4异六聚体以pH依赖性方式形成堆叠的十二聚体。我们的研究结果表明，CcmK 3-CcmK 4杂六聚体可能扩大的范围内的代谢物通道的渗透性能的羧基体壳。此外，所观察到的兼性形成的十二聚体在溶液中表明，羧基体壳的渗透性可以动态衰减的“加盖”面嵌入的六聚体与第二个六聚体。由于β-羧基体是专性表达的，异源六聚体的形成和加帽可以提供一种快速和可逆的方法来改变代谢物穿过外壳的通量，以响应环境/生长条件。
Bacterial microcompartments (BMCs) encapsulate enzymes within a selectively permeable, proteinaceous shell. Carboxysomes are BMCs containing ribulose-1,5-bisphosphate carboxylase oxygenase and carbonic anhydrase that enhance carbon dioxide fixation. The carboxysome shell consists of three structurally characterized protein types, each named after the oligomer they form: BMC-H (hexamer), BMC-P (pentamer), and BMC-T (trimer). These three protein types form cyclic homooligomers with pores at the center of symmetry that enable metabolite transport across the shell. Carboxysome shells contain multiple BMC-H paralogs, each with distinctly conserved residues surrounding the pore, which are assumed to be associated with specific metabolites. We studied the regulation of β-carboxysome shell composition by investigating the BMC-H genesccmK3andccmK4situated in a locus remote from other carboxysome genes. We made single and double deletion mutants ofccmK3andccmK4inSynechococcus elongatusPCC7942 and show that, unlike CcmK3, CcmK4 is necessary for optimal growth. In contrast to other CcmK proteins, CcmK3 does not form homohexamers; instead CcmK3 forms heterohexamers with CcmK4 with a 1:2 stoichiometry. The CcmK3-CcmK4 heterohexamers form stacked dodecamers in a pH-dependent manner. Our results indicate that CcmK3-CcmK4 heterohexamers potentially expand the range of permeability properties of metabolite channels in carboxysome shells. Moreover, the observed facultative formation of dodecamers in solution suggests that carboxysome shell permeability may be dynamically attenuated by “capping” facet-embedded hexamers with a second hexamer. Because β-carboxysomes are obligately expressed, heterohexamer formation and capping could provide a rapid and reversible means to alter metabolite flux across the shell in response to environmental/growth conditions.