Major envelope protein, Mep45, of 432 aas and 1 N-terminal TMS.  Mep45 contains two distinct domains: an N-terminal S-layer homologous (SLH) domain that protrudes into the periplasm and binds to peptidoglycan, and the remaining C-terminal transmembrane domain that forms a non-selective pore able to transport molecules of <600 Da (Kojima et al. 2016). The estimated pore radius is 0.58 nm; truncation of the SLH domain does not affect the channel.