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Structural Analysis of gp6 and gp6/gpl5/gpl6 Complex by Cryo-Electron Microscopy At 10 Å Resolution

The mechanism of encapsidation of DNA into a bacteriophage head is a most intriguing problem. The portal protein is essential for the assembly of tailed double-stranded DNA (dsDNA) bacteriophages. These turbine-like homo-oligomers consist of 12 or 13 subunits surrounding a central channel. Portal oligomers are located at the vertex of the icosahedral head that binds to the phage tail. It was shown that gp6 portal protein from bacteriophage SPP1 has 13 subunits, prior to their incorporation into the viral procapsid structure. After packaging of the viral DNA inside the phage capsid, additional proteins have been found attached to the portal oligomer: gpl5 and gp 16. A complex of gp6/gpl5/gpl7 forms the connector structure that provides the interface for attachment of the phage tail. We here present the three-dimensional (3D) reconstructions of the gp6 wild-type protein alone and of the gp6/gpl5/gpl6 complex - both at 10Å resolution - based on electron cryo-microscopy using the angular reconstitution single particle methodology.