2.A.1.2.6
(Benomyl, cycloheximide, methotrexate, fluconazole, etc.):H⁺ antiporter, CaMDR1 (Basso et al., 2010; Cannon et al., 1998). MDR1 catalyzes efflux of commonly used azoles. The central cytoplasmic loop is critical for MDR function, but does not impart substrate specificity (Mandal et al., 2012). The structural basis for polyspecificity of MDR MFS transporters, based on studies with Mdr1, is the extended capacity brought by residues located at the periphery of a binding core to accomodate compounds differing in size and type (Redhu et al. 2018). Each domain in the protein is arranged in a pseudo-symmetric fold of two tandems of 3-TMSs that alternatly expose the drug-binding site towards the inside or the outside of the yeast to promote drug binding and release. Sharma et al. 2022 provided information on these motifs by having screened a library of 64 drug transport-deficient mutants and their corresponding suppressors spontaneously addressing the deficiency. They found that five strains recovered the drug-resistance capacity by expressing CaMdr1 with a secondary mutation. The pairs of debilitating/rescuing residues are distributed either in the same TMS or 3-TMS repeat, at the hinge of 3-TMS repeat tandems, and between the N- and C-domains. Most of these mutants belong to different signature motifs, highlighting a mechanistic role and interplay thought to be conserved among MFS proteins. Results point to the specific role of TMS 11 in the interplay between the N- and C-domains in the inward- to outward-open conformational transition (Sharma et al. 2022).