Calcium Activation of the Ca-ATPase Enhances Conformational Heterogeneity between Nucleotide Binding and Phosphorylation Domains

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Abstract
High-resolution crystal structures obtained in two conformations of the Ca-ATPase suggest that a large-scale rigid-body domain reorientation of approximately 50° involving the nucleotide-binding (N) domain is required to permit the transfer of the γ-phosphoryl group of ATP to Asp351 in the phosphorylation (P) domain during coupled calcium transport. However, variability observed in the orientations of the N domain relative to the P domain in the different crystal structures of the Ca-ATPase following calcium activation and the structures of other P-type ATPases suggests the presence of conformational heterogeneity in solution, which may be modulated by contact interactions within the crystal. Therefore, to address the extent of conformational heterogeneity between these domains in solution, we have used fluorescence resonance energy transfer to measure the spatial separation and conformational heterogeneity between donor (i.e., 5-[[2-[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid) and acceptor (i.e., fluorescein 5-isothiocyanate) chromophores covalently bound to the P and N domains, respectively, within the Ca-ATPase stabilized in different enzymatic states associated with the transport cycle. In comparison to the unliganded enzyme, the spatial separation and conformational heterogeneity between these domains are unaffected by enzyme phosphorylation. However, calcium activation results in a 3.4 Å increase in the average spatial separation, from 29.4 to 32.8 Å, in good agreement with the 4.3 Å increase in the distance estimated from high-resolution structures where these sites are respectively separated by 31.6 Å (1IWO.pdb) and 35.9 Å (1EUL.pdb). Thus, the crystal structures accurately reflect the average solution structures of the Ca-ATPase. These results suggest that the approximation of cytoplasmic domains accompanying calcium transport, as observed from crystal structures, occurs in solution within the context of large amplitude domain motions important for catalysis. We suggest that these domain motions enhance the rates of substrate (ATP) access and product (ADP) egress and the probability of a productive juxtaposition of the γ-phosphoryl moiety of ATP with Asp351 on the phosphorylation domain to facilitate enzyme phosphorylation and calcium transport.