Tavares G A, Béguin P, Alzari P M
Unité d'Immunologie Structurale (URA 1961 CNRS), Paris, France.
J Mol Biol. 1997 Oct 31;273(3):701-13. doi: 10.1006/jmbi.1997.1326.
The quaternary organization of the cellulosome, a multi-enzymatic extracellular complex produced by cellulolytic bacteria, depends on specific interactions between dockerin domains, double EF-hand subunits carried by the catalytic components, and cohesin domains, individual receptor subunits linearly arranged within a non-catalytic scaffolding polypeptide. Cohesin-dockerin complexes with distinct specificities are also thought to mediate the attachment of cellulosomes to the cell membrane. We report here the crystal structure of a single cohesin domain from the scaffolding protein of Clostridium thermocellum. The cohesin domain folds into a nine-stranded beta-sandwich with an overall "jelly roll" topology, similar to that observed in bacterial cellulose-binding domains. Surface-exposed patches of conserved residues promote extensive intermolecular contacts in the crystal, and suggest a possible binding target for the EF-hand pair of the cognate dockerin domain. Comparative studies of cohesin domains indicate that, in spite of low sequence similarities and different functional roles, all cohesin domains share a common nine-stranded beta-barrel fold stabilized by a conserved hydrophobic core. The formation of stable cohesin-dockerin complexes requires the presence of Ca2+. However, the structure of the cohesin domain reported here reveals no obvious Ca2+-binding site, and previous experiments have failed to detect high affinity binding of Ca2+ to the unliganded dockerin domain of endoglucanase CelD. Based on structural and biochemical evidence, we propose a model of the cohesin-dockerin complex in which the dockerin domain requires complexation with its cohesin partner for protein stability and high-affinity Ca2+ binding.
纤维小体是由纤维素分解菌产生的一种多酶细胞外复合物，其四级结构取决于dockerin结构域（由催化组分携带的双EF-手亚基）与cohesin结构域（线性排列在非催化支架多肽中的单个受体亚基）之间的特异性相互作用。具有不同特异性的cohesin-dockerin复合物也被认为介导纤维小体与细胞膜的附着。我们在此报道了来自嗜热栖热放线菌支架蛋白的单个cohesin结构域的晶体结构。该cohesin结构域折叠成一个具有整体“果冻卷”拓扑结构的九股β-折叠三明治，类似于在细菌纤维素结合结构域中观察到的结构。保守残基的表面暴露斑块促进了晶体中的广泛分子间接触，并暗示了同源dockerin结构域的EF-手对的可能结合靶点。对cohesin结构域的比较研究表明，尽管序列相似性较低且功能作用不同，但所有cohesin结构域都共享一个由保守疏水核心稳定的共同九股β-桶折叠。稳定的cohesin-dockerin复合物的形成需要Ca2+的存在。然而，此处报道的cohesin结构域的结构未揭示明显的Ca2+结合位点，并且先前的实验未能检测到Ca2+与内切葡聚糖酶CelD的未结合dockerin结构域的高亲和力结合。基于结构和生化证据，我们提出了一个cohesin-dockerin复合物模型，其中dockerin结构域需要与其cohesin伙伴复合以实现蛋白质稳定性和高亲和力Ca2+结合。
