Enterobacterial phage T1 U-spanin, gp11, of 133 aas (Summer et al. 2007). It disrupts the host outer membrane and participates in cell lysis during virus exit. The spanin complex usually conducts the final step in host lysis by disrupting the outer membrane after holin and endolysin action have permeabilized the inner membrane and degraded the host peptidoglycans. Host outer membrane disruption is possibly due to local fusion between the inner and outer membrane performed by the spanin (Young 2014; Fernandes and São-José 2018). This spanin has been used for phage-based antimicrobial development (Yamashita et al. 2024).

gp11 mediates lysis in the absence of holin and endolysin function when the peptidoglycan density is depleted by starvation for murein precursors. Thus, the peptidoglycan is a negative regulator of gp11 function, supporting a model in which gp11 acts by fusing the inner and outer membranes, a mode of action analogous to but mechanistically distinct from that proposed for two-component spanin systems (Kongari et al. 2018).