Fig. 6.

Structure-guided functional dissection of MCR-3 colistin resistance.

A. Surface structure of MCR-3 with the cavity required for its PE lipid substrate entry.

B. An enlarged illustration for a five residues-containing, Zn²⁺-binding motif.

The five residues in Zn²⁺-binding motif of MCR-3 refers to E238, T277, H375, D450 and H451, respectively.

C. Surface architecture of MCR-3 in counter-clockwise rotation (30 °).

D. An enlarged view of the seven residues-containing motif that is involved in binding of MCR-3 to the PE lipid substrate.

The seven residues denote N103, T107, E111, G322, K325, H380 and H463, respectively.

E. Western blot analyses of the expression of MCR-3 and its 12 point-mutants in E. coli.

F. Structure-guided site-directed mutagenesis analyses for the Zn²⁺-binding motif of MCR-3 using the colistin susceptibility assays.

G. Functional mapping of the PE-interactive residues of MCR-3 in the context of colistin resistance.

H. The measurement of colistin MIC of the E. coli strains carrying the wild-type mcr-3 (and/or its point-mutants).

Level of colistin resistance was measured using the LBA plates. A representative result is given from no less than three independent trials.

In terms of level of colistin resistance, MCR-3 is almost identical to that of EptA, but only half of that seen with MCR-1.