1.C.129. The ΦX174 Lysis Protein E (ΦX174-E or PhiX174-E) Family
For the microviruses and the leviviruses, bacteriophages with small single-stranded genomes, host lysis is accomplished by expression of a single gene that encodes an inhibitor of cell wall synthesis. In contrast, phages with double-stranded DNA genomes use a more complex system involving, minimally, an endolysin, which degrades peptidoglycan, and a holin, which permeabilizes the membrane in a temporally programmed manner. A chimera was created in which lysis gene E of the microvirus phiX174 replaced the entire lysis cassette of phage lambda, which includes the holin gene S and the endolysin gene R. The chimeric phage was viable, but more variability was observed both in the distribution of plaque sizes and in the burst sizes of single cells, compared to the isogenic S(+) parent (Zheng et al. 2008). Using different alleles of E, it was found that the average burst size increased with the duration of the latent period, just as observed with S alleles with different lysis times. Moreover, within a set of missense E alleles, it was found that variability in lysis timing was limited and almost exclusively derived from changes in the level of E accumulation. By contrast, missense mutations in S resulted in a wide variation in lysis times that was not correlated with levels of accumulation. Zheng et al. (2008) suggested that the properties of greater phenotypic plasticity and lesser phenotypic variation make the function of holin proteins more genetically malleable, facilitating rapid adaptation towards a lysis time that would be optimal for changed host and environmental conditions. The inferior malleability of single-gene systems like E would restrict their occurrence to phages in which coding capacity is the overriding evolutionary constraint.
Vibrio anguillarum ghosts (VAG) were generated using a conjugation vector containing a ghost bacterial inducing cassette, pRK-λP(R)-cI-Elysis, in which the expression of ΦX174 lysis gene E was controlled by the P ( R )/cI regulatory system of λ phage. By scanning electron microscopy, holes ranging 80-200 nm in diameter were observed in the VAG (Kwon et al. 2009).
Phage ΦX174 protein E gene expression leads to host cell lysis by inhibition of the peptidoglycan synthesis enzyme MraY. Tanaka and Clemons (Tanaka and Clemons 2012) used mutagenesis to characterize the molecular details of the E lysis mechanism. They found that a minimal 18-residue region with the modified wild-type sequences of the conserved transmembrane helix of E was sufficient to lyse host cells, and that specific residues within and at the boundaries of this helix were important for activity. This suggests that positioning of the helix in the membrane is critical for interactions with MraY. They further characterized the interaction site of the transmembrane helix with MraY, demonstrating that E forms a stable complex with MraY (Tanaka and Clemons 2012).
The conformation of the soluble domain of PhiX174-E has been identified as a central trigger for membrane insertion, as well as for the oligomeric assembly of the toxin (Mezhyrova et al. 2020). Stable complex formation of the soluble domain with the SlyD chaparone protein is essential to keep nascent PhiX174-E in a conformation competent form for membrane insertion. Once inserted into the membrane, PhiX174-E assembles into high-order complexes via its transmembrane domain, and oligomerization depends on the presence of an essential proline residue at position 21. An initial contact of the nascent PhiX174-E TMD with the peptidyl-prolyl isomerase domain of SlyD is essential to allow a subsequent stable interaction of SlyD with the PhiX174-E soluble domain for the generation of a membrane insertion competent toxin (Mezhyrova et al. 2020).