1.W.2. The Phage Portal Protein 2 (PPP2) Family
The 90-nm-diameter capsid of coliphage T5 is organized with T=13 icosahedral geometry and encloses a double-stranded DNA genome that measures 121kbp. Its assembly follows a path similar to that of phage HK97 but yielding a larger structure that includes 775 subunits of the major head protein, 12 subunits of the portal protein and 120 subunits of the decoration protein. As for phage HK97, T5 encodes the scaffold function as an N-terminal extension (∆-domain) to the major head protein that is cleaved by the maturation protease after assembly of the initial prohead I form and prior to DNA packaging and capsid expansion. Although the major head protein alone is sufficient to assemble capsid-like particles, the yield is poor and includes many deformed structures. Huet et al. 2016 explored the role of both the portal and the protease in capsid assembly by generating constructs that include the major head protein and a combination of protease (wild type or an inactive mutant) and portal proteins and overexpressing them in E. coli. The results showed that the inactive protease mutant acts to trigger assembly of the major head protein, probably through binding to the ∆-domain, while the portal protein regulates assembly into the correct T=13 geometry. A cryo-electron microscopy reconstruction of prohead I including inactivated protease reveals density projecting from the prohead interior surface toward its center that is compatible with the ∆-domain, as well as additional internal density that has been assigned as the inactivated protease. These results reveal complexity in T5 beyond that of the HK97 system (Huet et al. 2016).