Spontaneous Suppressors against Debilitating Transmembrane Mutants of CaMdr1 Disclose Novel Interdomain Communication via Signature Motifs of the Major Facilitator Superfamily

The Major Facilitator Superfamily (MFS) drug:H(+) antiporter CaMdr1, from Candida albicans, is responsible for the efflux of structurally diverse antifungals. MFS members share a common fold of 12–14 transmembrane helices (TMHs) forming two N- and C-domains. Each domain is arranged in a pseudo-symmetric fold of two tandems of 3-TMHs that alternatively expose the drug-binding site towards the inside or the outside of the yeast to promote drug binding and release. MFS proteins show great diversity in primary structure and few conserved signature motifs, each thought to have a common function in the superfamily, although not yet clearly established. Here, we provide new information on these motifs by having screened a library of 64 drug transport-deficient mutants and their corresponding suppressors spontaneously addressing the deficiency. We 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 TMH (T127A(TMH1)- > G140D(TMH1)) or 3-TMHs repeat (F216A(TMH4)- > G260A(TMH5)), at the hinge of 3-TMHs repeats tandems (R184A(TMH3)- > D235H(TMH4), L480A(TMH10)- > A435T(TMH9)), and finally between the N- and C-domains (G230A(TMH4)- > P528H(TMH12)). Remarkably, most of these mutants belong to the different signature motifs, highlighting a mechanistic role and interplay thought to be conserved among MFS proteins. Results also point to the specific role of TMH11 in the interplay between the N- and C-domains in the inward- to outward-open conformational transition.