The structure of the nucleotide-binding domain of the Mg-ATPase MgtA from Escherichia coli has been solved and refined to 1.6 A resolution. The structure is made up of a six-stranded ,beta-sheet and a bundle of three alpha-helices, with the nucleotide-binding site sandwiched in between. The MgtA nucleotide-binding domain is shorter and more compact compared with that of the related Ca-ATPase and lacks one of the beta-strands at the edge of the ,beta-sheet. The ATP-binding pocket is surrounded by three sequence and structural motifs known from other P-type ATPases and a fourth unique motif that is found only in Mg-ATPases. This motif consists of a short polypeptide stretch running very close to the ATP-bindmg site, while in Ca-ATPase the binding site is more open, with the corresponding polypeptide segment folded away from the active site.
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