Dl. Stokes et Nm. Green, Modeling a dehalogenase fold into the 8-angstrom density map for Ca2+-ATPase defines a new domain structure, BIOPHYS J, 78(4), 2000, pp. 1765-1776
Members of the large family of P-type pumps use active transport to maintai
n gradients of a wide variety of cations across cellular membranes. Recent
structures of two P-type pumps at 8-Angstrom resolution have revealed the a
rrangement of transmembrane helices but were insufficient to reveal the arc
hitecture of the cytoplasmic domains. However, recent proposals of a struct
ural homology with a superfamily of hydrolases offer a new basis for modeli
ng these domains. In the current work, we have extended the sequence compar
ison for the superfamily and delineated domains in the 8-Angstrom density m
ap of Ca2+-ATPase. The homology suggests a new domain structure for Ca2+-AT
Pase and, specifically, that the phosphorylation domain adopts a Rossman fo
ld. Accordingly, the atomic structure of L-2 haloacid dehalogenase has been
fitted into the relevant domain of Ca2+-ATPase. The resulting model sugges
ts the existence of two ATP sites at the interface between two domains. Bas
ed on this new model, we are able to reconcile numerous results of mutagene
sis and chemical cross-linking within the catalytic domains. Furthermore, w
e have used the model to predict the configuration of MS ATP at its binding
site. Based on this prediction, we propose a mechanism,involving a change
in Mg2+ liganding, for initiating the domain movements that couple sites of
ion transport to ATP hydrolysis.