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 A resolution for both Ca2+-ATPase from rabbit sarcoplasmic reticulum and H+-ATPase from Neurospora crassa. These cation pumps are two distantly related members of the family of P-type ATPases, which are thought to use similar mechanisms to generate ATP-dependent ion gradients across a variety of cellular membranes. We have undertaken a detailed comparison of the two structures in order to describe their similarities and differences as they bear on their mechanism of active transport.


RESULTS
Our first important finding was that the arrangement of 10 transmembrane 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 the membrane-spanning region, the cytoplasmic regions of the two molecules were very different, both in their disposition relative to the membrane and in the juxtaposition of their various subdomains.


CONCLUSIONS
On the basis of the crystallization conditions, we propose that these two crystal structures represent different intermediates in the transport cycle, distinguished by whether cations are bound to their transport sites. Furthermore, we propose that the corresponding conformational change (E2 to E1 ) has two components: the first is an inclination of the main cytoplasmic mass by 20 degrees relative to the membrane-spanning domain; the 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 the membrane-spanning domain to the nucleotide-binding site, thus initiating the transport cycle.
Skirball Institute for Biomolecular Medicine, Department of Cell Biology, New York University School of Medicine, 540 First Avenue, New York, 10016, USA. stokes@saturn.med.nyu.edu
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D000818:Animals; D000252:Calcium-Transporting ATPases / Q000737:chemistry* / Q000502:physiology; D009492:Neurospora crassa; D011487:Protein Conformation; D017494:Proton Pumps / Q000737:chemistry* / Q000502:physiology; D006180:Proton-Translocating ATPases / Q000737:chemistry* / Q000502:physiology; D011817:Rabbits; D013329:Structure-Activity Relationship
D017494:Proton Pumps; D006180:Proton-Translocating ATPases; D000252:Calcium-Transporting ATPases