1.W.10. The (Enterobacterial Phage T7) Portal Protein 10 (PPP10) Family
The phage T7 portal protein has been structurually ellucidated at 8 Å resolution by cryo EM (Protein Databank acc # 6QWP_A; Agirrezabala et al. 2005). This protein forms the portal vertex of the capsid which plays critical roles in head assembly, genome packaging, neck/tail attachment, and genome ejection into the host bacterium. The portal protein multimerizes as a single ring-shaped homododecamer arranged around a central channel. The T7 connector is that of a 12-folded toroidal homopolymer with a channel that runs along the longitudinal axis of the particle. The structure of the T7 connector reveals many structural similarities with the connectors from other bacteriophages. Docking of the atomic structure of the varphi29 connector into the three-dimensional reconstruction of T7 connector reveals that the narrow, distal region of the two oligomers are almost identical. This region of the varphi29 connector is involved in DNA translocation, and is composed of an alpha-beta-alpha-beta-beta-alpha motif. A search for alpha-helices in the same region of the T7 three-dimensional map located three alpha-helices in approximately the same position as those of the varphi29 connector. A comparison of the predicted secondary structure of several bacteriophage connectors, including among others T7, varphi29, P22 and SPP1, reveals that, despite the lack of sequence homology, they seem to contain the same alpha-beta-alpha-beta-beta-alpha motif as that present in the varphi29 connector. These results suggest a common architecture related to a basic component of the DNA translocating machinery for several viruses (Agirrezabala et al. 2005).