Crystallization of the dl 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 a cross an energy-transducing membrane. Peripheral components of 380 and 200 residues bind NAD(H) (dI) and NADP(H) (dIII), respectively, while a third c omponent forms a membrane-spanning region (dII). The NAD(H)-binding compone nt dI of Rhodospirillum rubrum transhydrogenase has been crystallized in a form which diffracts to beyond 3.0 Angstrom resolution and is in space grou p P2 or P2(1), with unit-cell parameters a = 69.3, b = 117.8, c = 106.6 Ang strom, beta = 107.2 degrees and two dimers in the asymmetric unit. The sequ ence of the dI component is similar to that of alanine dehydrogenase. A ful l 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.