Functional expression in Pichia pastoris of human and rat intrinsic factor

Abstract

Intrinsic factor (IF) has been expressed previously in Baculovirus with a yield (0.1–1 mg/l) that was inadequate for structural and metabolic studies. IF cDNAs were cloned into the shuttle vector pPIC9 of P. pastoris, and the proteins were induced and purified by cobalamin (Cbl) affinity chromatography. Expression of recombinant proteins revealed a major band of 49 kDa for both human and rat IF. Expression of human IF was achieved at 10–40 mg/l, but of rat IF at only 1–2 mg/l. Reaction of human IF with a photo-activatable derivative of Cbl was demonstrated by Western blotting, and detection of IF fragments by anti-Cbl monoclonal antibody and by amino-terminal sequencing revealed at least three regions (residues 129–151, 234–254, and +294) linked to Cbl. Both recombinant human and rat [ 125I]IF–Cbl bound to rat and guinea pig brush border membranes with similar affinity, but the binding capacity of human IF for the rat receptor was only 10% compared with rat IF. All six amino acids within the previously identified N-terminal binding region of human IF were mutated to be identical to rat IF, but the resulting chimeric IF still bound poorly to rat membranes. Mutations of residues 26/27 (Glu 26 to Asp and Asn 27 to Gln) and 32/34 (Ser 32 to Thr and Tyr 34 to Arg) showed changes in both K a and V max, with great effects on V max. In conclusion, P. pastoris is an expression system that produces functional human IF at a higher yield than in the baculovirus system. Cbl binding was directly demonstrated at multiple sites along the linear sequence of human IF. The receptor binding function of the amino terminal sequence 25–62 has been confirmed, but it is insufficient to reproduce all the features of IF–Cbl binding.