Ion pumps have evolved independently several times during evolution. Their structure, mechanism of ion translocation and their energy source vary considerably [54]. Thus, various unrelated H÷ pumps can be energized by either light, redox energy, pyrophosphate or ATE H÷ pumps driven by ATP may consist of one (P-type) and up to at least 13 (FoF 1 from mitochondria) different subunits. P-type ATPases are ATP fuelled ion pumps with a single catalytic subunit, and have a phosphorylated reaction cycle intermediate [51]. The pumps become phosphorylated at an aspartate residue in the invariant sequence DKTGT [4]. At least two conformations exist, E~ and E 2, with conformational changes being accompanied by ion translocation [37]. Phosphorylation forces the enzyme into the E 2 conformation and following dephosphorylation the enzyme returns to the E 1 state. In addition to the catalytic subunit, one or two additional subunits may be present. P-type ATPases have been identified in almost all organisms studied thus far, the only current exception being the parasitic bacteria Borrelia burgdorferi (Table 1). P-type ATPases pump a variety of charged

: g Corresponding author. Fax:+ 45 35283333; E-mail: palmgren@ biobase. dk substrates such as K÷, Na÷, H÷, Mg 2÷, Ca 2÷, Cu 2÷, Cd 2÷ and phospholipids. The number of P-type ATPases in any organism varies considerably (Table 1); from very few in parasitic bacteria to seven to nine in free-living bacteria, 16 in the yeast Saccharomyces cerevisiae and probably more than 30 in plants and animals.