2.A.7.1.1
Small multidrug efflux pump, Smr (QacC, QacD, Ebr).  Substrates: (1) aromatic dyes (e.g., ethidium bromide), (2) quaternary amines (e.g., the disinfectant benzalkonium) and (3) derivatives of tetraphenylphosphonium (TPP) (Fuentes et al. 2005). Unassembled monomers of the SMR family exist in a dynamic equilibrium where the N-terminal TMS flips in and out of the membrane, with rates that depend on dimerization and the polypeptide sequence (Seurig et al. 2019). Additional substrates include guanidinium and metformin metabolites such as guanylurea (Lucero et al. 2023).