Preliminary X‐ray diffraction analysis of the cytoplasmic N‐terminal domain of the Na/HCO 3 cotransporter NBCe1‐A

The N‐terminal cytoplasmic domain of the Na + ‐coupled HCO 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 P 3 1 21 or P 3 1 with pseudo P 3 1 21 symmetry, with unit‐cell parameters a = 51.7, b = 51.7, c = 200.6 Å, α = β = 90, γ = 120°, and diffract X‐rays to 3.0 Å resolution. The calculated Matthews number is 1.9 Å 3  Da −1 , with two monomers of molecular weight ∼83 kDa in the asymmetric unit. The molecular‐ replacement packing solution shows that the molecules form dimers by a domain‐swapping mechanism.