Fig. 1. Structure and function of ArnT from Cupriavidus metallidurans CH34.

(A) Schematic representation of the reaction catalyzed by ArnT. The sugar L-Ara4N is transferred from the carrier UndP to lipid A. 1 and 4′ phosphate positions on lipid A are marked. (B) Analysis of ³²P-labeled lipid A species by TLC showing rescue of lipid A modification with L-Ara4N when ArnT_Cm is expressed in the ΔarnTΔeptA double-knockout E. coli strain (dm). The background E. coli strain (WD101; wt, wild type) has a pmrA constitutive phenotype (pmrA^C), enabling it to synthesize L-Ara4N- and pEtN-modified lipid A and to exhibit resistance to polymyxins (8). (C) Crystal structure of ArnT_Cm. Two orthogonal views, perpendicular to the plane of the membrane, are presented in ribbon representation with rainbow coloring starting from the N terminus (blue) to the C terminus (purple). The approximate dimensions of the monomer are 55,79, and 42 Å (width, height, and depth). Membrane boundaries shown were calculated using the PPM server (21). Dashed lines represent missing segments in the structure. (D) Arrangement of TM helices in ArnT_Cm shown as a slice of the TM domain at the level indicated by the arrow in (C). (E) Schematic representation of the connectivity and structural elements of ArnT_Cm. P, periplasm; M, membrane (inner); C, cytoplasm; TMD:TM domain; PL4 (shown in red).