1.K.3 The PhiX174 Tube-forming Spike Protein H (PhiX174-H) Family
Prokaryotic viruses have evolved various mechanisms to transport their genomes across bacterial cell envelopes. Many bacteriophages use a tail to perform this function, whereas tail-less phages rely on host organelles. However, the tail-less, icosahedral, single-stranded DNA PhiX174-like coliphages do not fall into these well-defined infection processes. For these phages, DNA delivery requires a DNA pilot protein. Sun et al. 2014 showed that the PhiX174 pilot protein H oligomerizes to form a tube whose function is probably to deliver the DNA genome across the host's envelope to the cytoplasm. The 2.4 A resolution crystal structure of the in vitro assembled H protein's central domain consists of a 170 A-long α-helical barrel. The tube is constructed of ten α-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. Genetic and biochemical studies demonstrated that the tube is essential for infectivity but does not affect in vivo virus assembly. Cryo-electron tomograms showed that tubes span the periplasmic space and are present while the genome is being delivered into the host cell's cytoplasm. Each end of the H protein contains a TMS, which anchors the assembled tubes into the inner and outer cell membranes. The central channel of the H-protein tube is lined with amide and guanidinium side chains. This may be a general property of viral DNA conduits and is likely to be critical for efficient genome translocation into the host (Sun et al. 2014).
The generalized reaction catalyzed by the H protein is:
Phage DNA (out) → Phage DNA (in).