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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",