Crystallization of the dI component of transhydrogenase, a proton-translocating membrane protein

Nicotinamide nucleotide transhydrogenase couples the exchange of a hydride-ion equivalent between NAD(H) and NADP(H) to the translocation of protons across an energy-transducing membrane. Peripheral components of 380 and 200 residues bind NAD(H) (dI) and NADP(H) (dIII), respectively, while a third component forms a membrane-spanning region (dII). The NAD(H)-binding component dI of Rhodospirillum rubrum transhydrogenase has been crystallized in a form which diffracts to beyond 3.0 Å resolution and is in space group P2 or P2₁, with unit-cell parameters a = 69.3, b = 117.8, c = 106.6 Å, β = 107.2° and two dimers in the asymmetric unit. The sequence of the dI component is similar to that of alanine dehydrogenase. A full structure determination will lead to important information on the mode of action of this proton pump and will permit the comparison of the structure–function relationships of dI with those of alanine dehydrogenase.