Scott W G, Milligan D L, Milburn M V, Privé G G, Yeh J, Koshland D E, Kim S H
Department of Chemistry, University of California, Berkeley 94720.
J Mol Biol. 1993 Jul 20;232(2):555-73. doi: 10.1006/jmbi.1993.1411.
The aspartate receptor is a transmembrane-signalling protein that mediates chemotaxis behaviour in bacteria. Aspartate receptors in Salmonella typhimurium and Escherichia coli exist as dimers of two subunits in the presence as well as in the absence of aspartate. We have previously reported the three-dimensional structures of the external ligand-binding domain of the S. typhimurium aspartate receptor with and without bound aspartate. The external or periplasmic region of the aspartate receptor is a dimer of four-alpha-helical bundle subunits; a single aspartate molecule binds to one of two sites residing at the subunit interface, increasing the affinity of the subunits for one another. Here we report the results of a detailed analysis of the aspartate receptor ligand-binding domain structure (residues 25 to 188). The dimer interface between the twofold related subunits consists primarily of contacts mediated by the side-chains of the N-terminal helix of each four-alpha-helical bundle subunit. The N-terminal helices pack approximately 20 degrees from parallel as an approximate coiled-coil super-secondary structure. We have refined aspartate receptor ligand-binding domain structures in the presence and in the absence of a bound aromatic compound, 1,10-phenanthroline, to 2.2 A and 2.3 A resolution, respectively, as well as crystal structures in the presence of specifically bound Au(I), Hg(II) and Pt(IV) complex ions at 2.4 A, 3.0 A and 3.3 A resolution, respectively. The possible biological relevance of the aromatic ligand-binding site and the metal ion-binding sites is discussed. The dimer of four-alpha-helical bundle subunits composing the periplasmic region of the S. typhimurium aspartate receptor provides a basis for understanding the results of mutational analyses performed on related chemotaxis transmembrane receptors. The crystal structure analysis provides an explanation for the way in which mutations in the E. coli aspartate receptor affect its binding to the periplasmic maltose-binding protein and how mutations in the more distantly related E. coli Trg chemotaxis receptor affect its binding to the periplasmic ribose and glucose-galactose binding proteins.
天冬氨酸受体是一种跨膜信号蛋白，介导细菌的趋化行为。鼠伤寒沙门氏菌和大肠杆菌中的天冬氨酸受体在有和没有天冬氨酸存在的情况下均以两个亚基的二聚体形式存在。我们之前报道了结合和未结合天冬氨酸的鼠伤寒沙门氏菌天冬氨酸受体外部配体结合结构域的三维结构。天冬氨酸受体的外部或周质区域是由四个α-螺旋束亚基组成的二聚体；单个天冬氨酸分子与位于亚基界面的两个位点之一结合，增加了亚基之间的亲和力。在此我们报告了对天冬氨酸受体配体结合结构域结构（第25至188位氨基酸残基）的详细分析结果。两个相关亚基之间的二聚体界面主要由每个四个α-螺旋束亚基的N端螺旋侧链介导的相互作用组成。N端螺旋以近似卷曲螺旋超二级结构的形式从平行方向大约倾斜20度堆积。我们分别将结合和未结合芳香族化合物1,10-菲咯啉的天冬氨酸受体配体结合结构域结构精修至2.2 Å和2.3 Å分辨率，以及将分别结合特异性结合的Au(I)、Hg(II)和Pt(IV)络合离子的晶体结构精修至2.4 Å、3.0 Å和3.3 Å分辨率。讨论了芳香族配体结合位点和金属离子结合位点可能的生物学相关性。构成鼠伤寒沙门氏菌天冬氨酸受体周质区域的四个α-螺旋束亚基二聚体为理解对相关趋化跨膜受体进行的突变分析结果提供了基础。晶体结构分析解释了大肠杆菌天冬氨酸受体中的突变如何影响其与周质麦芽糖结合蛋白的结合，以及更远缘相关的大肠杆菌Trg趋化受体中的突变如何影响其与周质核糖和葡萄糖 - 半乳糖结合蛋白的结合。
