Sequence, S-S bridges, and spectra of bovine transcobalamin expressed in Pichia pastoris

Transcobalamin (TC) -encoding cDNA was isolated from a bovine mammary gland cDNA library. Hybridization of the cloned bovine TC-cDNA to RNA samples from bovine tissues showed that the most intensive synthesis of a TC positive 1.9-kilobase mRNA occurred in kidney, lymphatic nodes, and liver. Bovine TC was expressed in yeast Pichia pastoris, and the isolated recombinant protein showed cobalamin (Cbl) and receptor binding properties similar to TCs from other sources. Alignment of the related Cbl carriers (haptocorrins and intrinsic factors from other species) with bovine TC (414 residues) revealed four conservative clusters in the sequence (85-98, 137-147, 178-190, and 268-288), which may be responsible for Cbl binding. Three S-S bonds connected Cys residues 3-252, 98-294, and 147-190. Treatment with an S-S reducing agent caused liberation of Cbl from TC-Cbl. A significant change was observed in the TC-Cbl absorbance spectrum upon substitution of Co(2+)-coordinated H(2)O by azide. The reaction developed several orders of magnitude slower, and the spectral distortions were much stronger than those in free Cbl. This may be caused by significant deformation of the Cbl molecule and/or by its shielding when bound to TC.