Comparison of H+-ATPase and Ca2+-ATPase suggests that a large conformational change initiates P-type ion pump reaction cycles

Background: Structures have recently been solved at 8 Angstrom resolution f or both Ca2+-ATPase from rabbit sarcoplasmic reticulum and Hf-ATPase from N eurospora crassa. These cation pumps are two distantly related members of t he family of P-type ATPases, which are thought to use similar mechanisms to generate ATP-dependent ion gradients across a variety of cellular membrane s. We have undertaken a detailed comparison of the two structures in order to describe their similarities and differences as they bear on their mechan ism of active transport. Results: Our first important finding was that the arrangement of 10 transme mbrane helices was remarkably similar in the two molecules. This structural homology strongly supports the notion that these pumps use the same basic mechanism to transport their respective ions. Despite this similarity in th e membrane-spanning region, the cytoplasmic regions of the two molecules we re very different, both in their disposition relative to the membrane and i n the juxtaposition of their various subdomains. Conclusions: On the basis of the crystallization conditions, we propose tha t these two crystal structures represent different intermediates in the tra nsport cycle, distinguished by whether cations are bound to their transport sites. Furthermore, we propose that the corresponding conformational chang e (E-2 to E-1) has two components: the first is an inclination of the main cytoplasmic mass by 20 degrees relative to the membrane-spanning domain; th e second is a rearrangement of the domains comprising the cytoplasmic part of the molecules. Accordingly, we present a rough model for this important conformational change, which relays the effects of cation binding within th e membrane-spanning domain to the nucleotide-binding site, thus initiating the transport cycle.