An Evolutionarily Conserved Family of Virion Tail Needles Related to Bacteriophage P22 gp26: Correlation between Structural Stability and Length of the α-Helical Trimeric Coiled Coil

Abstract

Bacteriophages of the Podoviridae family use short non-contractile tails to inject their genetic material into Gram-negative bacteria. In phage P22, the tail contains a thin needle, encoded by the phage gene 26, which is essential both for stabilization and ejection of the packaged viral genome. Bio-informatic analysis of the N-terminal domain of gp26 (residues 1–60) led us to identify a family of genes encoding putative homologues of the tail needle gp26. To validate this idea experimentally and to explore their diversity, we cloned the gp26-like gene from phages HK620, Sf6, HS1, and characterized these gene products in solution. All gp26-like factors contain an elongated α-helical coiled-coil core consisting of repeating, adjacent trimerization heptads and form trimeric fibers with length ranging between about 240Å to 300Å. Gp26-tail needles display high structural stability in solution, with Tm (temperature of melting) between 85–95°C. To determine how the structural stability of these phage fibers correlates with the length of the α-helical core, we investigated the effect of insertions and deletions in the helical core. In P22 tail needle, we identified an 85-residue long helical domain, termed MiCRU (Minimal Coiled-coil Repeat Unit), that can be inserted in frame inside gp26 helical core, preserving the straight morphology of the fiber. Likewise, we were able to remove three quarters of the helical core of HS1 tail needle minimally decreasing the stability of the fiber. We conclude that in the gp26 family of tail needles, structural stability increases non-linearly with the length of the α-helical core. Thus, the overall stability of these bacteriophage fibers is not solely dependent on the number of trimerization repeats in the α-helical core.

Atomic Structure of Bacteriophage Sf6 Tail Needle Knob

“…Like the other component proteins of the tail, the tail needle gp26 (9) is essential for viability (11). Gp26 functions as a molecular "plug" that seals the portal protein channel after DNA packaging to stabilize viral DNA in the capsid (11,12). Accordingly, disrupting the gene encoding gp26 results in non-infective particles that have lost their DNA (11).…”

“…All tail needles characterized so far consist of trimeric fibers ϳ220 -320 Å in length 3 (12). Trimerization is mediated by a long trimeric coiled-coil core that contains 11-16 trimerization heptads (12).…”

“…All tail needles characterized so far consist of trimeric fibers ϳ220 -320 Å in length 3 (12). Trimerization is mediated by a long trimeric coiled-coil core that contains 11-16 trimerization heptads (12). As revealed by the crystal structure of P22 gp26, the tail needle is only 20 -25 Å in diameter (14,15) and possesses remarkably high structural stability (16).…”

“…As revealed by the crystal structure of P22 gp26, the tail needle is only 20 -25 Å in diameter (14,15) and possesses remarkably high structural stability (16). The tail needle N-terminal ϳ60 residues are highly conserved in all P22-like phages, providing an attachment point for gp10 and serving to plug the portal channel (12,14,16). In contrast, the tail needle C terminus is variable both in sequence and morphology; it represents the most exposed moiety of the virion distal to the head (6) and is thought to be involved in penetrating the host.…”

“…In contrast, the tail needle C terminus is variable both in sequence and morphology; it represents the most exposed moiety of the virion distal to the head (6) and is thought to be involved in penetrating the host. The C-terminal domain is helical in P22 (14,15) and HK620 (12) but forms a globular "knob" in other phages such as Sf6 and HS1 (12), which infect the Gram-negative bacteria Shigella flexneri and Escherichia coli, respectively.…”