Figure 10-5: Holin function and the dual start motif.  Holins and antiholins from dual-start holin genes are depicted as light and dark shaded rectangles, respectively.  The cartoon reflects the normal 2:1 proportion of the holin S105 and S107 and assumes that dimerization is the first step in hole formation.  According to this model, each S107, which preferentially dimerizes with S105 (56), removes one S105 from the pool of holin monomers involved in the timing of hole formation.  At the instant of the first hole formation, the collapse of the membrane potential allows the S107 protein to assume the conformation of the S105 holin, converting the heterodimers to functional holin dimers.  Thus, the scheduled triggering results in tripling the amount of functional holin dimers available for hole formation.  Adapted from Gründling et al. (56), with permission.