Structure, transmembrane topology and helix packing of P-type ion pumps

Electron microscopy has recently provided improved structures for P-type ion pumps. In the case of Ca 2+-ATPase, the use of unstained specimens revealed the structure of the transmembrane domain. The composition of this domain has been controversial due to the variety of methods used to study the number and exact locations of transmembrane crossings within the sequence. After reviewing the results from several members of the family, we found a consensus for 10 transmembrane segments, and also that 10 helices fitted well into the structure of Ca 2+-ATPase. Thus, we present the most detailed model for transmembrane structure so far, in the hope of stimulating more precise experimental strategies.