Structural studies of the maltose transporter

ATP-binding cassette (ABC) transporters are ubiquitous membrane proteins that import or export a large variety of materials ranging from ions to lipids and proteins. In prokaryotes, these proteins are critical survival and virulence factors. In Humans, there are 48 different ABC transporters on board, and more than a dozen genetic diseases have been traced to ABC transporter defects. ABC transporters are also central to multidrug resistance in many pathogenic bacteria and tumor cells. We exploit the maltose transporter from E. coli as a model system to analyze how ABC transporters convert chemical energy from ATP hydrolysis to mechanical work for substrate translocation. The maltose transporter is composed of two TM subunits, MalF and MalG, two subunits of a cytoplasmic ATPase, MalK, and a periplasmic substrate-binding protein, MBP, required for the ATPase activity of the transporter. Using X-ray crystallography, we captured the maltose transporter in multiple conformations along the transport cycle. We show that in the absence of transport substrate and nucleotide, the transporter forms an inward-facing conformation with its TM maltose-binding site exposed to the cytoplasm. Interactions with substrate-loaded MBP in the periplasm induce a partial closure of the MalK dimer in the cytoplasm, to which ATP binding then promotes progression from the inward-facing to the outward-facing state. Formation of the outward-facing state enables hydrolysis by orienting the g-phosphate and a hydrolytic water molecule. These structures, interpreted in the light of decades of functional data, show how the chemical event of ATP hydrolysis is linked to global conformational changes of the transporter.