An MsbA deletion mutant DeltaC21 that lacks the two C-terminal alpha-helices was expressed in Escherichia coli strain C41 and purified by metal-affinity and gel-filtration chromatography. Purified DeltaC21 retained 26% of the activity of the wild-type ATPase and had a similar binding affinity to fluorescent nucleotide derivatives. Although crystals of wild-type MsbA complexed with adenosine 5'-(beta,gamma-imido)triphosphate could not be obtained, crystals of DeltaC21 that diffracted to 4.5 A resolution were obtained. The preliminary DeltaC21 structure had the outward-facing conformation, in contrast to the previously reported E. coli MsbA structure. This result suggests that deletion of the C-terminal alpha-helices may play a role in facilitating the outward-facing nucleotide-bound crystal structure of EcMsbA.