Molecular architecture of the prolate head of bacteriophage T4

Abstract

The head of bacteriophage T4 is a prolate icosahedron with one unique portal vertex to which the phage tail is attached. The three-dimensional structure of mature bacteriophage T4 head has been determined to 22-A resolution by using cryo-electron microscopy. The T4 capsid has a hexagonal surface lattice characterized by the triangulation numbers T(end) = 13 laevo for the icosahedral caps and T(mid) = 20 for the midsection. Hexamers of the major capsid protein gene product (gp)23* and pentamers of the vertex protein gp24*, as well as the outer surface proteins highly antigenic outer capsid protein (hoc) and small outer capsid protein (soc), are clearly evident in the reconstruction. The size and shape of the gp23* hexamers are similar to the major capsid protein organization of bacteriophage HK97. The binding sites and shape of the hoc and soc proteins have been established by analysis of the soc(-) and hoc(-)soc(-) T4 structures.

Fig. 1.

Structure of the bacteriophage T4 head (T_end = 13 laevo, T_mid = 20, h₁ = 3, k₁ = 1, h₂ = 4, and k₂ = 2) compared with the previously proposed model (T_end = 13 laevo, T_mid = 21, h₁ = 3, k₁ = 1, h₂ = 3, and k₂ = 3) (4, 9). The facet triangles are shown in blue and the basic triangles (see Fig. 4) are shown in black as appropriate. (A) Shaded surface representation of the cryo-EM reconstruction viewed perpendicular to the fivefold axis. gp23* is shown in blue, gp24* is in magenta, soc is in white, hoc is in yellow, and the tail is in green. (B) Model of the previously proposed T4 head structure (adapted from ref. 4). (C) View of the reconstruction along the fivefold axis with the portal vertex toward the observer; the tail has been cut away at the level shown by the black arrow in A. Proteins are colored as described for A. (D, Left) Schematic representation of the distribution of proteins in the elongated midsection facet. (D, Right) Schematic representation of an end-cap facet. Proteins are colored as described for A except the soc molecules are shown as gray rectangles. A and C were prepared with the help of the programs dino (www.dino3d.org) and povray (www.povray.org).

Fig. 2.

Shaded surface representations of the soc^– (A) and hoc^–soc^– (B) reconstructions. A facet triangle is shown.

Fig. 3.

The central section of the reconstructed cryo-EM density map viewed from a direction perpendicular to the fivefold axis. The concentric layers beneath the outer capsid shell are attributed to the densely packaged dsDNA. The spacing between successive layers is ≈25 Å. The high-density region in the capsid interior next to the portal vertex is attributed to the gp20 connector (see the enlargement in Inset).

Fig. 4.

Surface lattice organization for a prolate bacteriophage T4 head. Small triangles represent the “basic” triangles. Each such triangle contains three asymmetric subunits schematically shown in gray (top left). The large equilateral triangle corresponds to an end-cap facet. It is defined by one vertex at the origin and another vertex at (h₁ = 3, k₁ = 1) and corresponds to a T_end = 13 laevo lattice. The large dashed and solid elongated triangles describe the midsection facets. One vertex of these triangles is at the origin, another is at the point (h₁ = 3, k₁ = 1), and the third is at (h₂, k₂). The dashed triangle corresponds to the midsection structure of the previously proposed bacteriophage T4 model (4, 9), whereas the solid triangle corresponds to the cryo-EM reconstruction reported here.