Figure 23-6: Three-dimensional structures of the bacteriophage SPP1 portal protein G6P and of the connector. The structures are based on data from electron micrographs of G6P oligomers and connectors imaged under the electron microscope and processed by single particle analysis (94). A shows different views of the G6P cyclical 13-mer and a cut-open view along the symmetry axis of the molecule (bottom right). In B are shown equivalent views of the connector composed of stacked rings of G6P, G15P, and G16P. Each ring is composed of 12 subunits. The wide region of the connector formed by G6P is oriented towards the interior of the viral capsid while the bottom is exposed to the phage tail. The connector internal channel that can accommodate a DNA molecule is closed at the level of G16P (bottom ring) preventing exit of DNA from the phage capsid (Courtesy of EV Orlova).