Crystal structure of a phosphorylation-coupled saccharide transporter

Abstract

Saccharides have a central role in the nutrition of all living organisms. Whereas several saccharide uptake systems are shared between the different phylogenetic kingdoms, the phosphoenolpyruvate-dependent phosphotransferase system exists almost exclusively in bacteria. This multi-component system includes an integral membrane protein EIIC that transports saccharides and assists in their phosphorylation. Here we present the crystal structure of an EIIC from Bacillus cereus that transports diacetylchitobiose. The EIIC is a homodimer, with an expansive interface formed between the amino-terminal halves of the two protomers. The carboxy-terminal half of each protomer has a large binding pocket that contains a diacetylchitobiose, which is occluded from both sides of the membrane with its site of phosphorylation near the conserved His 250 and Glu 334 residues. The structure shows the architecture of this important class of transporters, identifies the determinants of substrate binding and phosphorylation, and provides a framework for understanding the mechanism of sugar translocation. Most living organisms metabolize saccharides to generate energy. Cell membranes are impermeable to saccharides, so specialized transport mechanisms are vital. The main saccharide transport mechanism in bacteria is the phosphoenolpyruvate-dependent phosphotransferase system. Zhou and colleagues have solved the X-ray crystal structure of the integral membrane protein EIIC, a phosphorylation-coupled saccharide transporter, from Bacillus cereus. EIIC is the first member of the extremely large phosphotransferase superfamily of proteins for which a structure has been determined. The protein has a novel structural fold and the structure, which is co-crystallized with a diacetylchitobiose, reveals details of the substrate binding site and residues likely to be involved in phosphorylation.