1.K.4. The Phage P22 Injectisome (P22 Injectisome) 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.
Phage P22 of Salmonella typhimurium ejects four proteins, gp7, gp16, gp20 and gp26. These proteins are ejected from the phage virion into the bacterial cell after absorption. Mutants lacking gp7 or gp20 still eject the other proteins, but gp16- cells do not eject gp7 (Israel, 1977). These four proteins may play a role in DNA ejection.
gp20 is a soluble protein of 457 aas with no strikingly hydrophobic regions, but several strong peaks of amphipathicity, three within residues 70-170, and two more at residues 350 and 400 when the angle is set at 100° for an α-helix. gp16 (609 aas) exhibits three N-terminal peaks of hydrophobicity (residues 70-190) plus one C-terminal peak at residue position 550. It is membrane bound and could form the transmembrane channel. gp26 (P35837) is a packaged DNA stabilization protein, and gp7, like gp16 and gp20, is a DNA transfer protein.
gp16 alone forms channels in lipid bilayers that transport fluorescent dyes and DNA (Perez et al. 2009). Moreover, gp7 and gp20 accompany the phage DNA into the E. coli cytoplasm and may be required for DNA cyclization to form circular DNA from linear DNA. All three of these proteins are basic and bind DNA nonspecifically.
The proton motive force is required for DNA entry into the cell, and possibly, the positively charged proteins, coating the DNA, neutralize the negative charges in the DNA, allowing the nucleoprotein complex to enter the cytoplasm by electrophoresis. gp26 may play a role in transport of the complex across the outer membrane, possibly by forming a channel or serving as a receptor.
The generalized reaction catalyzed by the injectisome is:
DNAout (in phage capsid) → DNA (in bacterial cell cytoplasm)