In this article we will discuss about the structure of phage M13.
M13 (or f1) phage is a filamentous phage that shows entirely alpha-helical in structure so that the molecule forms a short rod. The helical capsid is simplest measuring about 900 nm long and 9 nm in diameter. M13 is encapsulated in about 2700 copies of the major coat protein pVIII, and capped with 5 copies of two different minor coat proteins (pIX, pXI, pill) at the ends.
The minor coat protein PHI gets attached to the receptor at the tip of the F pilus of E. coli. (Fig. 18.23). Ff phage particles are held together by the hydrophobic interactions between the coat protein subunits. Within the capsid it encloses a 6407 nucleotides long, circular, single- stranded DNA which possesses 10 genes in the genome. It infects E. coli through pili and produces new virions without lysing the host cell.
The product of phage gene 8 (pVIII) is the major coat protein and there are 2,700-3,000 copies of this protein per particle. About 5 copies each of four minor capsid proteins (i.e. pill, pVI, pVII and pIX) are located at the ends of the filamentous particle.
Mature molecules of pVIII consist of about 50 amino acid residues. A signal sequence of 23 amino acids is cleaved from the precursor protein during its translocation into the outer membrane of the host bacterium.
There are three distinct domains within this rod (Fig. 18.23):
First domain is a negatively-charged region at the amino terminal end contains acidic amino acid residues and forms the outer hydrophilic surface of the virus particle.
Second domain is a basic, positively-charged region at the carboxyl terminal end lines the inside of the protein cylinder adjacent to the negatively-charged DNA genome.
Third domain is a hydrophobic region responsible for interactions between the pVIII subunits which stabilize the phage particle.
The pVIII subunits in successive turns of the helix interlock with the subunits in the turn below, are tilted at an angle of approximately 20° to the long axis of the particle and have been described as overlapping one another ‘like the scales of a fish’. The value of µ (protein subunits per complete helix turn) is 4.5 and p (axial rise per subunit) = 1.5nm.