This family represents T4 phage gp25 protein and gp25-like proteins, found in several systems, including contractile tail bacteriophages, the type VI secretion system (T6SS) and R-type pyocins, which constitute a multiprotein tubular apparatus to attach to and penetrate host cell membranes. Gp25 is a component of the conserved wedge in the inner part of the baseplate and serves as a nucleus for sheath polymerisation, playing a critical role in sheath assembly and contraction [1,2]. The EPR motif (Glu-Pro-Arg, residues 85-87 of gp25) is conserved across all members of the family including orthologues from the RpoS-mediated general stress response system (called IraD) [3,4]. This motif interacts with the 'core bundle' composed of orthologues of T4 gp6 and gp7 proteins in contractile injection systems.

1: Structure of the T4 baseplate and its function in triggering sheath contraction. Taylor NM, Prokhorov NS, Guerrero-Ferreira RC, Shneider MM, Browning C, Goldie KN, Stahlberg H, Leiman PG; Nature. 2016;533:346-352. PMID:27193680

2: Structure-function analysis of HsiF, a gp25-like component of the type VI secretion system, in Pseudomonas aeruginosa. Lossi NS, Dajani R, Freemont P, Filloux A; Microbiology. 2011;157:3292-3305. PMID:21873404

3: The RpoS-mediated general stress response in Escherichia coli. Battesti A, Majdalani N, Gottesman S; Annu Rev Microbiol. 2011;65:189-213. PMID:21639793

4: Structural basis for inhibition of a response regulator of sigma(S) stability by a ClpXP antiadaptor. Dorich V, Brugger C, Tripathi A, Hoskins JR, Tong S, Suhanovsky MM, Sastry A, Wickner S, Gottesman S, Deaconescu AM; Genes Dev. 2019;33:718-732. PMID:30975721