Architecture and Principle of Formation of the Baseplate of Bacteriophage T4

尾溶菌酶gp5的晶体结构已测定到2.9 a的分辨率。gp5是基板结构上必不可少的组成部分，也具有溶菌酶活性。将其c端带有His-Tag的Gp5与Ni-NTA琼脂糖凝胶结合，柱作为亲和色谱仪分离分离制备的Seleno-Met取代gp27， gp27是基板″hub″的另一组分，用于相测定。从Ni-NTA琼脂糖柱分离得到的配合物亚基组成为（gp5*）^3(gp5C)^3(gp27)^3，其中gp5*为成熟的尾溶菌酶，gp5C为c端结构域。该复合物的整体形状呈“火炬”状，其中三聚体gp27和n端结构域形成杯状，溶菌酶结构域形成边缘，c端结构域呈“茎”状。gp5的三个结构域由两个具有44个残基的连接子（连接子1）和49个残基的连接子（连接子2）连接。n端结构域具有一个所谓的OB基序（OB代表寡核苷酸/寡糖结合），它是一个五链β -桶。溶菌酶结构域与T4溶菌酶相似，rms为1.1 A。c端结构域形成长110a、宽28a的长棱镜。c端结构域的n端46个残基形成5链反平行β-片。每个亚基的三个薄片形成一个短棱镜。剩下的部分形成三股β-螺旋。β-螺旋的横截面显示了来自每个多肽链的三个相同的VXGXXXX序列，在甘氨酸残基处形成一个结，形成每个三角形的顶点。β链形成平行的β片。β-螺旋是一种非常稳定的结构，在室温下可以抵抗SDS变性。

Crystal structure of the tail lysozyme, gp5, which is a structurally essential component of the baseplate and also possesses lysozyme activity has been determined to the resolution of 2.9 A. Gp5 of which the C-terminus had His-Tag was bound to Ni-NTA agarose gel and the column was then used as an affinity chromatograph to isolate separately prepared Seleno-Met substituted gp27 which is another component of the ″hub″of the baseplate for phase determination.Isolated complex from the Ni-NTA agarose column had the subunit composition of (gp5*)^3(gp5C)^3(gp27)^3, where gp5* is the mature tail lysozyme, gp5C is the C-terminal domain. The overall shape of the complex appears a "torch", where trimer gp27 and the N-terminal domain form the cup, the lysozyme domain forms the rim, and the C-terminal domain assumes the "stem". The three domains of gp5 are connected by two linkers with 44 residues (linker 1) and 49 residues (linker 2). The N-terminal domain possesses a so-called OB motif (OB stands for oligonucleotide/oligosaccharide binding) which is a five-stranded beta-barrel. The lysozyme domain is similar to T4 lysozyme with the rms of 1.1 A. The C-terminal domain forms a long prism with the length of 110 A and the width of 28 A. The N-terminal 46 residues in the C-terminal domain form 5-stranded anti-parallel β-sheets. The three beta sheets, from each subunit, form a short prism. The remaining part forms a triple-stranded β-helix. The cross-section of the β-helix reveals three identical sequence of VXGXXXX from each polypeptide chain with a kink at glycine residue which forms the vertice of each triangle. The β-strands form parallel beta sheets. The β-helix is a very stable structure which is resistant to denaturation by SDS at room temperature.