This model represents a protein found encoded in F1F0-ATPase operons in several genomes, including Methanosarcina barkeri (archaeal) and Chlorobium tepidum (bacterial). It is a small protein (about 100 amino acids) with long hydrophobic stretches and is presumed to be a subunit of the enzyme. It carries two transmembrane helices and is a magnesium or calcium uniporter. The atp operon of alkaliphilic Bacillus pseudofirmus OF4, as in most prokaryotes, contains the eight structural genes for the F-ATPase (ATP synthase), which are preceded by an atpI gene that encodes a membrane protein with 2 TMSs. A tenth gene, atpZ, has been found in this operon, which is upstream of and overlapping with atpI [1].

1: A tenth atp gene and the conserved atpI gene of a Bacillus atp operon have a role in Mg2+ uptake. Hicks DB, Wang Z, Wei Y, Kent R, Guffanti AA, Banciu H, Bechhofer DH, Krulwich TA; Proc Natl Acad Sci U S A. 2003;100:10213-10218. PMID:12917488