The resulting holin monomers are then inserted into

the cell membrane, where they dimerize, then oligomerize [37], eventually leading to the formation of higher-order holin aggregates, or rafts, in the cell membrane. At a time that is specific to the holin protein sequence, the holin rafts are transformed into a membrane lesion(s) > 300 nm across [38], which is large enough for the passage of a 500 KDa protein [28, 29]. Lysis ensues after endolysin digests the peptidoglycan. Thus, by regulating endolysin’s access to the peptidoglycan, holin controls the timing of lysis [26, 27]. To formalize the heuristic model of holin hole formation described by Wang et al. [28], Ryan and Rutenberg [39] proposed a two-stage nucleation model, in which the production rate of the holin monomers and holin self-affinity contribute to the aggregation of holin rafts. Raft aggregation is opposed by thermal Brownian LGX818 in vitro motion which tends to disintegrate rafts into their holin constituents. As the rafts grow and then exceed a certain critical size (the first stage of nucleation), the probability of a second stage nucleation (triggering to hole formation) increases (Figure 1). According to this model, lysis time stochasticity is the inevitable outcome of each infected cell in the population following its own time course of growth in holin raft size. However, a recent study [40] using C-terminus GFP-fused

λ S holin protein showed Megestrol Acetate that, for most of the latent period, holin Tariquidar proteins are distributed uniformly in a relatively mobile state in the cell membrane. At a time that coincided with the triggering AZD6738 time, large immobile holin rafts suddenly appeared in the membrane. The transition from uniformly distributed holin to holin rafts occurred in less than a minute. Although it is not clear whether these large rafts correspond to the membrane holes observed by cryoelectron microscopy [38], this study nevertheless casts doubt on the previously hypothesized importance of holin raft size growth as the determining factor in lysis timing [28, 39]. Rather, it is proposed that

the lysis time is determined by when a critical holin concentration is reached in the cell membrane (Figure 1). According to this model, lysis time stochasticity is mainly the result of variation in the timing of reaching the critical holin concentration in the membrane. Figure 1 Schematic presentation of two models of holin hole formation. Holin monomers (shaded circles) are produced in the cytoplasm, and then transported to the cell membrane (a top-down view of the cell membrane thereafter) where they dimerize. A previous model (open arrows) [28, 39] hypothesized that the growth of the holin aggregates (“”rafts”") to a critical size that is responsible for the collapse of the proton motive force (pmf), thus resulting in hole formation.