Information theoretic measures and mutagenesis identify a novel linchpin residue involved in substrate selection within the nucleotide-binding domain of an ABCG family exporter Cdr1p.

ABC transporters are membrane-bound pumps composed of two major domains: the transmembrane domain(s) (TMDs) and the nucleotide-binding domain(s) (NBDs). Sequence analyses of the NBDs of key ABC exporters revealed a residue position within the H-loop to be differentially conserved in the ABCG family, wherein there lies glutamine instead of positively charged arginine/lysine as in non-ABCG members. Consequently, contrasting NBD sequences of fungal Pleiotropic Drug Resistance transporters (PDR/ABCG) with that of Cholesterol/Phospholipid and Retinal (CPR/ABCA) Flippase family revealed a high Cumulative Relative Entropy (CRE) score of this residue position implying its family-specific functional significance. Further, substitution of the glutamine by arginine in both the NBDs of a representative PDR/ABCG member, (Candida drug resistance 1 protein) Cdr1p led to selective susceptibility of the Saccharomyces cerevisiae strains overexpressing the corresponding mutant proteins (Q362R and Q1060R) towards antifungal substrates without any impact on the ATPase activity. Consistent with the findings from previous studies on H-loop motif of fungal PDR transporters, the current report points towards a role of the glutamine residue within both canonical and divergent H-loop of Cdr1p in conferring substrate selection in a precisely identical manner.