FixI is a pump of a specific cation involved in symbiotic nitrogen fixation. The four proteins FixG, FixH, FixI, and FixS may participate in a membrane-bound complex coupling the FixI cation pump with a redox process catalyzed by FixG. ATP + H2O = ADP + H(+) + phosphate Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IB subfamily.