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Researchers from Berkeley Lab and the University of California, Berkeley, have used the Macromolecular Crystallography Facility at the ALS to obtain the crystal structure of the HisP subunit of histidine permease, an ABC transporter from Salmonella typhimurium. ABC transporters variously act as pumps that propel substances through membranes, as channels that allow the substances' passage, and as regulators that control other membrane proteins. In S. typhimurium, histidine permease contains two identical nucleotide-binding domains, known as HisP, that together form a dimer. The structure of the HisP monomer exhibits an unusual L shape with two thick arms, one of which holds the ATP binding site and the other of which is proposed to be in contact with a membrane-spanning domain. It is possible that binding and hydrolysis of ATP in one arm results in conformational changes in the other arm that are passed on to the membrane-spanning domain, but the mechanism is not yet known in any detail.