Department of Biology, The Catholic University of America, Washington, DC 20064, USA.
Mol Microbiol. 2010 Jul;77(2):444-55. doi: 10.1111/j.1365-2958.2010.07219.x. Epub 2010 May 19.
Bacteriophage T4 is decorated with 155 copies of the highly antigenic outer capsid protein, Hoc. The Hoc molecule (40 kDa) is present at the centre of each hexameric capsomer and provides a good platform for surface display of pathogen antigens. Biochemical and modelling studies show that Hoc consists of a string of four domains, three immunoglobulin (Ig)-like and one non-Ig domain at the C-terminus. Biochemical data suggest that the Hoc protein has two functional modules, a capsid binding module containing domains 1 and 4 and a solvent-exposed module containing domains 2 and 3. This model is consistent with the dumbbell-shaped cryo-EM density of Hoc observed in the reconstruction of the T4 capsid. Mutagenesis localized the capsid binding site to the C-terminal 25 amino acids, which are predicted to form two beta-strands flanking a capsid binding loop. Mutations in the loop residues, ESRNG, abolished capsid binding, suggesting that these residues might interact with the major capsid protein, gp23*. With the conserved capsid binding module forming a foothold on the virus and the solvent-exposed module able to adapt to bind to a variety of surfaces, Hoc probably provides survival advantages to the phage, such as increasing the virus concentration near the host, efficient dispersion of the virus and exposing the tail for more efficient contact with the host cell surface prior to infection.
T4 噬菌体表面装饰有 155 个高度抗原性的外壳蛋白 Hoc。Hoc 分子（40kDa）位于每个六聚体壳粒的中心，为病原体抗原的表面展示提供了良好的平台。生化和建模研究表明，Hoc 由四个结构域组成，三个免疫球蛋白（Ig）样结构域和一个 C 末端的非 Ig 结构域。生化数据表明，Hoc 蛋白有两个功能模块，一个包含结构域 1 和 4 的衣壳结合模块和一个包含结构域 2 和 3 的暴露在溶剂中的模块。该模型与 T4 衣壳重建中观察到的 Hoc 的哑铃形冷冻电镜密度一致。突变定位了衣壳结合位点到 C 末端的 25 个氨基酸残基，这些残基预测形成两个β-折叠，侧翼有一个衣壳结合环。环残基 ESRNG 的突变使衣壳结合丧失，表明这些残基可能与主要衣壳蛋白 gp23*相互作用。由于保守的衣壳结合模块在病毒上形成立足点，而暴露在溶剂中的模块能够适应与各种表面结合，因此 Hoc 可能为噬菌体提供了生存优势，例如增加宿主附近的病毒浓度、有效分散病毒以及在感染前暴露出尾部以更有效地与宿主细胞表面接触。
