1.K.4 The Phage P22 Cell Envelope-penetrating Needle (P22-CEN) Family
Bacteriophage P22 infects Salmonella enterica by injecting its genetic material through the cell envelope. During infection, a specialized tail needle, gp26 (233aas), is injected into the host, likely piercing a hole in the host cell envelope (Olia et al., 2007). The 2.1-A crystal structure of gp26 reveals a 240-A elongated protein fiber formed by two trimeric coiled-coil domains interrupted by a triple beta-helix. The N terminus of gp26 plugs the portal protein channel, retaining the genetic material inside the virion. The C-terminal tip of the fiber exposes beta-hairpins with hydrophobic tips similar to those seen in class II fusion peptides. The alpha-helical core connecting these two functionally polarized tips presents four trimerization octads with consensus sequence IXXLXXXV. The slender conformation of the gp26 fiber minimizes the surface exposed to solvent, which is consistent with the idea that gp26 traverses the cell envelope lipid bilayers (Olia et al., 2007). Homologues gp26 are found in many phage, and even within large eukaryotic viral proteins where the corresponding sequences can be repeated several times. They are sometimes referred to as 'packaged DNA stabilization proteins'. They are usually between 200 and 300aas in size.
The generalized reaction suggested for gp26 is:
DNA (phage head) → DNA (bacterial cell)