Bagnéris C, Bateman O A, Naylor C E, Cronin N, Boelens W C, Keep N H, Slingsby C
Department of Crystallography, Birkbeck College, Institute of Structural and Molecular Biology, Malet Street, London WC1E 7HX, UK.
J Mol Biol. 2009 Oct 9;392(5):1242-52. doi: 10.1016/j.jmb.2009.07.069. Epub 2009 Jul 30.
Small heat shock proteins (sHsps) are a family of large and dynamic oligomers highly expressed in long-lived cells of muscle, lens and brain. Several family members are upregulated during stress, and some are strongly cytoprotective. Their polydispersity has hindered high-resolution structure analyses, particularly for vertebrate sHsps. Here, crystal structures of excised alpha-crystallin domain from rat Hsp20 and that from human alphaB-crystallin show that they form homodimers with a shared groove at the interface by extending a beta sheet. However, the two dimers differ in the register of their interfaces. The dimers have empty pockets that in large assemblies will likely be filled by hydrophobic sequence motifs from partner chains. In the Hsp20 dimer, the shared groove is partially filled by peptide in polyproline II conformation. Structural homology with other sHsp crystal structures indicates that in full-length chains the groove is likely filled by an N-terminal extension. Inside the groove is a symmetry-related functionally important arginine that is mutated, or its equivalent, in family members in a range of neuromuscular diseases and cataract. Analyses of residues within the groove of the alphaB-crystallin interface show that it has a high density of positive charges. The disease mutant R120G alpha-crystallin domain dimer was found to be more stable at acidic pH, suggesting that the mutation affects the normal dynamics of sHsp assembly. The structures provide a starting point for modelling higher assembly by defining the spatial locations of grooves and pockets in a basic dimeric assembly unit. The structures provide a high-resolution view of a candidate functional state of an sHsp that could bind non-native client proteins or specific components from cytoprotective pathways. The empty pockets and groove provide a starting model for designing drugs to inhibit those sHsps that have a negative effect on cancer treatment.
小分子热休克蛋白（sHsps）是一类大型动态寡聚体家族，在肌肉、晶状体和大脑的长寿细胞中高度表达。几个家族成员在应激期间上调，有些具有强大的细胞保护作用。它们的多分散性阻碍了高分辨率结构分析，尤其是对于脊椎动物的sHsps。在这里，大鼠Hsp20和人αB-晶状体蛋白切除的α-晶状体蛋白结构域的晶体结构表明，它们通过延伸β折叠在界面处形成具有共享凹槽的同型二聚体。然而，这两种二聚体在界面的配准上有所不同。二聚体有一些空穴，在大型组装体中可能会被来自伙伴链的疏水序列基序填充。在Hsp20二聚体中，共享凹槽部分被处于多聚脯氨酸II构象的肽填充。与其他sHsp晶体结构的结构同源性表明，在全长链中，凹槽可能被N端延伸填充。凹槽内部是一个与对称相关的功能重要的精氨酸，在一系列神经肌肉疾病和白内障的家族成员中发生了突变或其等效突变。对αB-晶状体蛋白界面凹槽内残基的分析表明，它具有高密度的正电荷。发现疾病突变体R120Gα-晶状体蛋白结构域二聚体在酸性pH下更稳定，这表明该突变影响了sHsp组装的正常动力学。这些结构通过定义基本二聚体组装单元中凹槽和空穴的空间位置，为更高层次组装的建模提供了一个起点。这些结构提供了一个sHsp候选功能状态的高分辨率视图，该状态可以结合非天然客户蛋白或细胞保护途径的特定成分。空穴和凹槽为设计抑制那些对癌症治疗有负面影响的sHsps的药物提供了一个起始模型。
