Architecture and Principle of Formation of the Baseplate of Bacteriophage T4

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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.