Mapping the pinhole formation pathway of S21

Phage holins are small, lethal membrane proteins of two general types: canonical holins, like λ S105, which oligomerizes and forms large membrane holes of unprecedented size; and pinholins, like S(21) 68 of lambdoid phage 21, which forms homo-heptameric channels, or pinholes, with a lumen of <2 nm. Pinholes depolarize the membrane, leading to activation of secreted endolysins and murein degradation. S(21) 68 has two transmembrane domains, TMD1 and TMD2. TMD2 alone lines the pinhole, making heterotypic interactions involving two surfaces, A and B. Mutational analysis on S(21) 68 suggested that S(21) 68 initially forms inactive dimer, with TMD1 inhibiting TMD2 both in cis and trans. When TMD1 exits the membrane to the periplasm, it liberates TMD2 to participate in the pathway to pinhole formation. In this study, further mutational analysis suggests a refined pinhole formation pathway, with the existence of at least two intermediate states. We propose that the pathway begins in the activated dimer state, with a homotypic TMD2 interface involving the A surface. Evidence is presented for a further oligomeric state involving a heterotypic A:B interaction. Moreover, the data suggest that a glycine-zipper motif present in the A interface of TMD2 is involved in every stage downstream of the inactive dimer.