The nitrate reductase of higher plants, algae, and fungi is a homodimeric cytosolic protein with five conserved domains in each monomer: 1) an Mo-MPT domain that contains the single molybdopterin cofactor, 2) a dimer interface domain, 3) a cytochrome b domain, and 4) an NADH domain that combines with 5) an FAD  domain to form the cytochrome b reductase fragment. There exists a GPI-anchored variant that is found on the outer face of the plasma membrane. Its exact function is still not clear.