Abstract
Proteins of the proton-pumping pyrophosphatase (H+-PPase) family are found in the vacuolar (tonoplast) membranes of higher plants, algae and protozoa, and in both bacteria and archaea (Baltscheffsky & Baltscheffsky, 1993; Baltscheffsky et al., 1999; Drozdowicz et al., 1999; Kim et al., 1994, 1995; Sarafian et al., 1992a, b). They are therefore presumed to be ancient enzymes, having arisen before the divergence of the three domains of life (Drozdowicz & Rea, 2001). The plant, algal and archaeal enzymes, of which there may be several isoforms in a single organism, probably pump one H+ upon hydrolysis of pyrophosphate, thereby generating a proton motive force, positive and acidic in the tonoplast lumen or negative and basic in the cytoplasm of a prokaryote (Bäumer et al., 2002; Drozdowicz et al., 2003; Moriyama et al., 2003). These enzymes, some of which are phosphate-starvation-inducible, establish a pmf of similar magnitude to that generated by the H+-translocating ATPases in the same membranes (Blumwald et al., 2000; Moriyama et al., 2003; Motta et al., 2004; Palma et al., 2000; Ruiz et al., 2001; Zhen et al., 1997a, b). The bacterial and archaeal proteins may catalyse fully reversible reactions, thus being able to synthesize pyrophosphate when the pmf is sufficient (Bäumer et al., 2002; Belogurov et al., 2002; Drozdowicz et al., 1999). The enzyme from Rhodospirillum rubrum is induced in response to a number of environmental stress conditions and contributes to the pmf when light intensity is insufficient to generate a pmf that can support rapid ATP synthesis (García-Contreras et al., 2004; López-Marqués et al., 2004).