Transmission of E1-E2 structural changes in response to Na+ or K+ binding in Na,K-ATPase

The extensive E(1)-E(2) conformational changes in response to Na(+) or K(+) binding in the absence of other ligands must be driven by motion of the side chains contributing to cation coordination, but the differences in structure of Na(+) and K(+) sites have not been resolved. The recent high resolution structure model of the E(2) conformation of Ca-ATPase offers the first opportunity to examine and model the changes accompanying the adjustment of the cation sites from an E(1) form with specificity for Na(+) to an E(2) form with specificity for K(+). The model of the E(2) form provides a remarkable fit to the data of direct Tl(+) or K(+) binding after site-directed mutagenesis of residues Asp804 and Asp808 in M6, Glu 779, Gln776, and Ser775 in M5, and Glu327 in M4. Cytoplasmic domain movements during E(1) <--> E(2) conformational transition can be monitored by proteolytic cleavage. Protection of the chymotrypsin-sensitive bond at Leu266 in L2/3 and rotation of the A domain is more complete in the E(2)Mg-vanadate-ouabain complex than in the E(2)[2K] form.