Characterization of an Integral Protein of the Brush Border Membrane Mediating the Transport of Divalent Metal Ions

Abstract

The transport of Fe 2+ and other divalent transition metal ions across the intestinal brush border membrane (BBM) was investigated using brush border membrane vesicles (BBMVs) as a model. This transport is an energy-independent, protein-mediated process. The divalent metal ion transporter of the BBM is a spanning protein, very likely a protein channel, that senses the phase transition of the BBM, as indicated by a break in the Arrhenius plot. The transporter has a broad substrate range that includes Mn 2+, Fe 2+, Co 2+, Ni 2+, Cu 2+, and Zn 2+. Under physiological conditions the transport of divalent metal ions is proton-coupled, leading to the acidification of the internal cavity of BBMVs. The divalent metal ion transporter can be solubilized in excess detergent (30 mM diheptanoylphosphatidylcholine or 1% Triton X-100) and reconstituted into an artificial membrane system by detergent removal. The reconstituted membrane system showed metal ion transport characteristics similar to those of the original BBMVs. The properties of the protein described here closely resemble those of the proton-coupled divalent cation transporter (DCT1, Nramp2) described by Gunshin et al. (1997, Nature. 388:482–488). We may conclude that a protein of the Nramp family is present in the BBM, facilitating the transport of Fe 2+ and other divalent transition metal ions.