This region is occasionally found in conjunction with Pfam:PF03335. Most of the family appear to be phage tail proteins; however some appear to be involved in other processes. For instance Swiss:Q03314 from Rhizobium leguminosarum may be involved in plant-microbe interactions ([1]). A related protein Swiss:Q9L3N1 is involved in the pathogenicity of Microcystis aeruginosa. The finding of this family in a structural component of the phage tail fibre baseplate (Swiss:P10930) suggests that its function is structural rather than enzymatic. Structural studies show this region consists of a helix and a loop ([2]) and three beta-strands. This alignment does not catch the third strand as it is separated from the rest of the structure by around 100 residues. This strand is conserved in homologues but the intervening sequence is not. Much of the function of Swiss:P10930 appears to reside in this intervening region. In the tertiary structure of the phage baseplate this domain forms part of the 'collar'. The domain may bind SO4, however the residues accredited with this vary between the PDB file and the Swiss-Prot entry. The long unconserved region maybe due to domain swapping in and out of a loop or reflective of rapid evolution.

1: Molecular characterization and regulation of the rhizosphere-expressed genes rhiABCR that can influence nodulation by Rhizobium leguminosarum biovar viciae. Cubo MT, Economou A, Murphy G, Johnston AW, Downie JA; J Bacteriol 1992;174:4026-4035. PMID:1597418

2: The structure of the receptor-binding domain of the bacteriophage T4 short tail fibre reveals a knitted trimeric metal-binding fold. Thomassen E, Gielen G, Schutz M, Schoehn G, Abrahams JP, Miller S, van Raaij MJ; J Mol Biol 2003;331:361-373. PMID:12888344