A single molecule of H protein is found on each of the 12 spikes on the microvirus shell. H is involved in the ejection of the phage DNA, and at least one copy is injected into the host's periplasmic space along with the ssDNA viral genome [1]. Part of H is thought to lie outside the shell, where it recognises lipopolysaccharide from virus-sensitive strains [2]. Part of H may lie within the capsid, since mutations in H can influence the DNA ejection mechanism by affecting the DNA-protein interactions [3]. H may span the capsid through the hydrophilic channels formed by G proteins [1]. Elucidation of the DNA-ejection mechanism from the crystal structure of part of the H protein shows that this tail-less icosahedral, single-stranded DNA phiX174-like coliphage bacteriophage requires H as a pilot protein for its DNA-delivery. H oligomerises to form a tube the function of which seems to be the delivery of the DNA genome across the host's periplasmic space into the host cytoplasm. The tube is constructed of ten alpha-helices with their amino termini arrayed in a right-handed super-helical coiled-coil and their carboxy termini arrayed in a left-handed super-helical coiled-coil. The tube spans the periplasmic space and is present while the genome is being delivered into the host cell's cytoplasm [4].

1: Analysis of the single-stranded DNA bacteriophage phi X174, refined at a resolution of 3.0 A. McKenna R, Ilag LL, Rossmann MG; J Mol Biol 1994;237:517-543. PMID:8158636

2: Specific interaction of fused H protein of bacteriophage phiX174 with receptor lipopolysaccharides. Suzuki R, Inagaki M, Karita S, Kawaura T, Kato M, Nishikawa S, Kashimura N, Morita J; Virus Res 1999;60:95-99. PMID:10225278

3: Role of DNA-protein interactions in bacteriophage phi X174 DNA injection. Ilag LL, Tuech JK, Beisner LA, Sumrada RA, Incardona NL; J Mol Biol 1993;229:671-684. PMID:8433365

4: Icosahedral bacteriophage PhiX174 forms a tail for DNA transport during infection. Sun L, Young LN, Zhang X, Boudko SP, Fokine A, Zbornik E, Roznowski AP, Molineux IJ, Rossmann MG, Fane BA; Nature. 2013; [Epub ahead of print] PMID:24336205