Department of Biochemistry, University of California, Riverside, 2482B Boyce Hall, Riverside, CA 92521-0123, USA.
J Mol Biol. 2011 Apr 22;408(1):74-86. doi: 10.1016/j.jmb.2011.02.004. Epub 2011 Feb 15.
Small heat shock proteins (sHsps) are ubiquitous low-molecular-weight chaperones that prevent protein aggregation under cellular stresses. sHsps contain a structurally conserved α-crystallin domain (ACD) of about 100 amino acid residues flanked by varied N- and C-terminal extensions and usually exist as oligomers. Oligomerization is important for the biological functions of most sHsps. However, the active oligomeric states of sHsps are not defined yet. We present here crystal structures (up to 1.65 Å resolution) of the sHspA from the plant pathogen Xanthomonas (XaHspA). XaHspA forms closed or open trimers of dimers (hexamers) in crystals but exists predominantly as 36mers in solution as estimated by size-exclusion chromatography. The XaHspA monomer structures mainly consist of α-crystallin domain with disordered N- and C-terminal extensions, indicating that the extensions are flexible and not essential for the formation of dimers and 36mers. Under reducing conditions where α-lactalbumin (LA) unfolds and aggregates, XaHspA 36mers formed complexes with one LA per XaHspA dimer. Based on XaHspA dimer-dimer interactions observed in crystals, we propose that XaHspA 36mers have four possible conformations, but only XaHspA 36merB, which is formed by open hexamers in 12mer-6mer-6mer-12mer with protruding dimers accessible for substrate (unfolding protein) binding, can bind to 18 reduced LA molecules. Together, our results unravel the structural basis of an active sHsp oligomer.
小分子热休克蛋白（sHsps）是普遍存在的低分子量伴侣蛋白，可在细胞应激下防止蛋白质聚集。sHsps 含有一个结构保守的约 100 个氨基酸残基的 α-晶体蛋白结构域（ACD），两侧为不同的 N-和 C-末端延伸，通常以寡聚体形式存在。寡聚化对于大多数 sHsps 的生物学功能很重要。然而，sHsps 的活性寡聚状态尚未确定。我们在此介绍植物病原体黄单胞菌（Xanthomonas）sHspA 的晶体结构（最高分辨率为 1.65 Å）。XaHspA 在晶体中形成封闭或开放的二聚体三聚体（六聚体），但在溶液中主要以 36 聚体存在，这是通过凝胶过滤色谱法估计的。XaHspA 单体结构主要由 α-晶体蛋白结构域和无序的 N-和 C-末端延伸组成，这表明延伸是灵活的，对于二聚体和 36 聚体的形成不是必需的。在还原条件下，α-乳白蛋白（LA）展开并聚集，XaHspA 36 聚体与每个 XaHspA 二聚体形成一个 LA 复合物。基于在晶体中观察到的 XaHspA 二聚体-二聚体相互作用，我们提出 XaHspA 36 聚体有四种可能的构象，但只有 XaHspA 36merB 可以与 18 个还原 LA 分子结合，XaHspA 36merB 是由开放的六聚体在 12mer-6mer-6mer-12mer 中形成的，其中突出的二聚体可用于底物（展开蛋白）结合。总之，我们的结果揭示了活性 sHsp 寡聚体的结构基础。
