Slingsby C, Bateman O A
Department of Crystallography, Birkbeck College, London, U.K.
Biochemistry. 1990 Jul 17;29(28):6592-9. doi: 10.1021/bi00480a007.
beta-Crystallins are complex eye lens proteins made up of several related basic and acidic subunits that combine to form differently sized oligomers each displaying extensive polydispersity. As the sequences are homologous to the X-ray-determined bilobal structure of gamma-crystallin, beta-subunits are visualized as having a similar structure with additional N- and C-terminal extensions. Two basic (beta B2 and beta B3) and two acidic (beta A3 and beta A4) subunits have been isolated in deaggregating media, refolded, and reassociated in various combinations to determine which components favor dimers or higher oligomers. Homopolymers were compared with beta B2 homodimer in terms of charge, using Mono Q fast protein liquid chromatography, and size, using Superose 12 chromatography. Heterooligomeric formations were monitored by their intermediate charge properties compared with homooligomers. beta B2 associates with either beta B3- or beta A4-forming heterodimers whereas a larger oligomer is formed with beta A3. Naturally occurring beta-crystallin oligomers were analyzed by Mono Q chromatography and PhastGel electrophoresis. Whereas beta B2, beta B3, and beta A4 can each be reassociated to homodimers, beta A4 dimers are not found in native beta-crystallins. beta B2-beta A3 is a major component of intermediate-sized beta L1-crystallin and is absent from dimeric beta L2-crystallin. It is suggested that the pH dependence of the size of beta L1-crystallin is due to a dimer to tetramer equilibrium. By following dimer interactions using Superose 12 chromatography, beta B2-beta A4 was shown to interact with beta B2-beta A3. A model of beta-crystallin structure is proposed based on beta-subunits forming dimers with the next level of organization requiring an acidic subunit, beta A3, with a long N-terminal extension.
β-晶状体蛋白是复杂的眼晶状体蛋白，由几个相关的碱性和酸性亚基组成，这些亚基结合形成大小不同的寡聚体，每个寡聚体都表现出广泛的多分散性。由于这些序列与通过X射线确定的γ-晶状体蛋白的双叶结构同源，β-亚基被视为具有类似的结构，并带有额外的N端和C端延伸。在解聚介质中分离出了两个碱性亚基（βB2和βB3）和两个酸性亚基（βA3和βA4），对其进行重折叠，并以各种组合重新缔合，以确定哪些组分有利于形成二聚体或更高阶的寡聚体。使用Mono Q快速蛋白质液相色谱法比较了同聚物与βB2同二聚体的电荷，并使用Superose 12色谱法比较了它们的大小。通过与同聚体比较其中间电荷性质来监测异源寡聚体的形成。βB2与βB3或βA4缔合形成异二聚体，而与βA3形成更大的寡聚体。通过Mono Q色谱法和PhastGel电泳分析了天然存在的β-晶状体蛋白寡聚体。虽然βB2、βB3和βA4各自都可以重新缔合形成同二聚体，但在天然β-晶状体蛋白中未发现βA4二聚体。βB2-βA3是中等大小的βL1-晶状体蛋白的主要成分，而二聚体βL2-晶状体蛋白中不存在该成分。有人认为，βL1-晶状体蛋白大小对pH的依赖性是由于二聚体与四聚体的平衡。通过使用Superose 12色谱法跟踪二聚体相互作用，发现βB2-βA4与βB2-βA3相互作用。基于β-亚基形成二聚体，而更高层次的组织需要一个带有长N端延伸的酸性亚基βA3，提出了β-晶状体蛋白结构模型。
