Preliminary X-ray diffraction analysis of the cytoplasmic N-terminal domain of the Na/HCO3 cotransporter NBCe1-A

The N-terminal cytoplasmic domain of the Na+- coupled HCO3- cotransporter NBCe1-A (NtNBCe1) has been linked with proximal renal tubular acidosis. In a previous purification study of recombinant NtNBCe1, crystal growth at a suboptimal protein concentration (< 1 mg ml(-1)) yielded small single diamond-shaped crystals that diffracted poorly. In the present study, by increasing the protein concentration 50- fold, the crystal size was doubled and robustness was also improved. Crystal annealing made the crystals suitable for X-ray diffraction. The crystals either belong to space group P3(1)21 or P3(1) with pseudo P3(1)21 symmetry, with unit-cell parameters a = 51.7, b = 51.7, c = 200.6 angstrom, alpha = ss = 90, gamma = 120 degrees, and diffract X-rays to 3.0 A resolution. The calculated Matthews number is 1.9 angstrom(3) Da(-1), with two monomers of molecular weight similar to 83 kDa in the asymmetric unit. The molecular-replacement packing solution shows that the molecules form dimers by a domain-swapping mechanism.