Crystal Structure of the Carboxy-Terminal Region of the Bacteriophage T4 Proximal Long Tail Fiber Protein Gp34

Abstract

Long tail fibers of bacteriophage T4 are formed by proteins gp34, gp35, gp36, and gp37, with gp34 located at the phage-proximal end and gp37 at the phage-distal, receptor-binding end. We have solved the structure of the carboxy-terminal region of gp34, consisting of amino acids 894-1289, by single-wavelength anomalous diffraction and extended the structure to amino acids 744-1289 using data collected from crystals containing longer gp34-fragments. The structure reveals three repeats of a mixed α-β fibrous domain in residues 744 to 877. A triple-helical neck connects to an extended triple β-helix domain (amino acids 900-1127) punctuated by two β-prism domains. Next, a β-prism domain decorated with short helices and extended β-helices is present (residues 1146-1238), while the C-terminal end is capped with another short β-helical region and three β-hairpins. The structure provides insight into the stability of the fibrous gp34 protein.

Keywords: Caudovirales; Myoviridae; bacterial viruses; crystallography; fibrous protein.

Figure 1

Schematic drawing of the bacteriophage T4 long tail fiber. The baseplate-binding proximal end is indicated with a B, the receptor-binding distal end with an R. The component proteins are indicated with colors: gp34 in red, gp35 in green, gp35 in blue, and gp37 in yellow. The carboxy-terminal end of gp34 is indicated with an asterisk. The numbers inside the red gp34 outline indicate where the P3, P4, and P5 domains are located. A box indicates the approximate size of the largest fragment for which the structure was solved (residues 744–1289).

Figure 2

Crystallographic structure of the carboxy-terminal part of gp34. (a) Cartoon view of the structure of gp34(744–1289). Approximate domain boundaries are indicated. (b) Space-filled representation of the structure in the same orientation as A, illustrating the extensive intertwining of the three monomers in the trimer. (c) Topology diagram of the structure. The termini, as well as the starts and ends of most of the secondary structure elements, are labelled.

Figure 3

Structure comparison of N-terminal domains and repeating sequences of gp34. (a) Side view of the trimeric structure of one of the amino-terminal domains, residues 745–776. (b) Top view of (a). (c) Superposition of the monomeric structure of the three amino-terminal repeats of gp34(744–776 in cyan, 780–812 in red, 823–868 in green) and one repeat of gp12(182–215) in yellow. (d) Alignment of repeating sequences of gp34 and gp12.

Figure 4

Triangular domains in bacteriophage fibers and tailspikes. (a) Cross-section of the P4 domain, viewed from the N-terminal end. (b) Side view of the P5 domain. One of the β-hairpins is indicated with an asterisk. The β-strands of the central β-sheet are numbered. Note that the fifth strand is irregular. (c) Side view of the bacteriophage P22 tailspike, which is a trimer of gp9. The triangular domain in this and the next panel are boxed and the termini are labelled. (d) Side view of the C-terminal part of the bacteriophage T7 tail fiber (gp17). In panels (c) and (d), N- and C-termini are labelled with Nt and Ct, respectively.

Figure 5

Fitting of the gp34(744–1289) crystal structure in cryo-electron microscopy density of the bacteriophage T4 tail. (a) Crystal structure in red fitted into EM density of the T4 tail (EMD-1126, grey). (b) Close view of (a). White arrows indicate connected EM density between long tail fiber and phage sheath. (c) Representation of local three-fold axis of the C-terminal part (blue, 882–1289) and N-terminal part (red, 744–881).