If indeed some apparently icosahedral viruses have special vertices, then it would be relevant to examine the impact of special vertices on the structure and function of the well studied tailed phages. Bacteriophage T4 has a total length of about 2200 Å, with a prolate head about 1200 Å in length and 850 Å in width containing a genome of 172 kbp coding for about 300 genes. There are about 40 separate proteins in the assembled virion, many of them in multiple copies. The tail terminates with a 425 Å wide hexagonally shaped baseplate to which are attached six long and six short tail fibers. The long tail fibers recognize Escherichia coli and thereby initiate contraction of the sheath around the tail tube, forcing the tail tube through the center of the baseplate into the E. coli periplasmic space. The tail tube terminates in a pin-like structure (a trimer of gene product 5; gp5) surrounded by three lysozyme domains (each a segment of the three gp5 polypeptides) that digest the peptidoglycan cell wall, resulting in the injection of the T4 genome into the host bacterium. This machine is highly efficient, producing an infection almost every time a phage particle interacts with a recognizable E. coli cell. The success of T4 and other tailed phages depends on having a specialized vertex to which is attached the complex tail organelle. In contrast, viruses that depend on eukaryotic host cells often require 50 or even 100 virions per cell for successful infection. Thus, possibly, some icosahedral viruses that infect eukaryotic cells might increase their infection efficiency, even in the absence of a tail, by having a special vertex to aid genome delivery to the host.
The icosahedral ends of bacteriophage T4 heads have T
= 13 quasi-symmetry and the cylindrical mid-section has Q
= 20 quasi-symmetry (Fokine et al.
). The protein shell of the mature T4 capsid is formed by the major capsid protein gp23, the special vertex protein gp24, the highly antigenic outer capsid protein (hoc), the small outer capsid protein (soc) and the head–tail connector gp20. Cryo-EM reconstructions have identified the location of these proteins in the head (Fig. 5) (Fokine et al.
). The major capsid protein forms a hexagonal array of hexamers. The soc proteins bind to the interface between gp23 hexamers.
Figure 5 Cryo-EM reconstruction of the head capsid of bacteriophage T4, based on fivefold symmetry averaging. The major capsid protein (gp23, in blue) forms hexamers. The small outer capsid protein (soc, in white) binds between gp23 hexamers. The highly antigenic (more ...)
Of special interest is the vertex protein gp24. It makes pentamers which form 11 of the 12 capsid vertices. The 12th vertex is the unique portal vertex occupied by the gp20 dodecamer. The portal vertex also provides the binding site for the DNA-packaging machine consisting of gp17, gp16 and gp20 that functions to package the genomic DNA into the pre-assembled prohead. Once the DNA is packaged, the tail is attached to form a fully assembled infectious virus particle.
The polypeptide fold of gp24 (Fokine et al.
) is similar to the major capsid protein of the tailed phage HK97 (Wikoff et al.
). The sequence of gp24 is homologous to that of gp23 (about 21% identical amino acids), making it possible to build a homology model of gp23. Furthermore, the comparison with HK97 provided the basis of building gp24 pentamers and gp23 hexamers. These could in turn be fitted into the cryo-EM density to make a pseudo-atomic model for a substantial part of the head capsid. The similarity between the T4 and HK97 capsid architectures makes a strong case that both these virus capsids evolved from a common primordial phage head, as is also the case for the tailed phages P22 (Jiang et al.
), ϕ29 (Morais et al.
) and epsilon 15 (Jiang et al.
The T4 phage head consists of hexagonally packed planes of gp23 hexamers. The fivefold icosahedral vertices occur at the intersection of these planes, requiring a special pentagonal vertex protein. Apparently, the primordial phage had only one type of protein, but with gene duplication the specialized protein evolved independently of the major capsid protein gp23. A reversion to the primordial phage is produced by ‘bypass’ mutations in which the absence of the gp24 gene is compensated by mutations in gp23 (Fokine et al.
; Fig. 6)
Figure 6 Structure of the pentameric vertex of the T4 gp24 bypass mutant viewed down the fivefold axis of the capsid. The soc molecules are colored grey. The additional soc molecules bound around the gp23* pentamers are marked in red. Reprinted from Fokine et (more ...)