Natural resistance to intracellular infections: Nramp1 encodes a membrane phosphoglycoprotein absent in macrophages from susceptible (Nramp1 D169) mouse strains.

Abstract

The mouse Nramp1 gene (Bcg/Ity/Lsh) controls innate defense to infection with intracellular parasites such as Mycobacterium, Salmonella, and Leishmania. Sequence analysis of Nramp1 predicts a hydrophobic, membrane-associated protein expressed exclusively in monocyte/macrophage lineages. A single G169D substitution within the fourth predicted transmembrane domain of Nramp1 is associated with susceptibility to infection (Bcg) in inbred mouse strains. To initiate the biochemical characterization of the Nramp1 protein and to analyze the molecular basis of susceptibility associated with the Nramp1(D169) allele, oligopeptides derived from Nramp1 and two fusion proteins containing the first 54 and the last 35 residues of Nramp1 were used to raise specific anti-Nramp1 antisera. In addition, a c-Myc epitope (EQKLISEEDL) was introduced in-frame at the C terminus of Nramp1 to follow its expression in a yeast heterologous system. Western analysis of crude membrane fractions from yeast demonstrated that Nramp1 is indeed an integral membrane protein (resistant to urea extraction). In resident Bcg(r) (129sv, Nramp1(G169)) macrophages, one of the anti-Nramp1 antisera identified by immunoprecipitation a protein of 90,000 to 100,000 apparent m.w. that was absent in macrophages from knockout mice bearing a null mutation at Nramp1. The Nramp1 protein could be metabolically labeled with orthophosphate and was sensitive to glycosylase treatment, verifying that Nramp1 is an integral membrane phosphoglycoprotein. Parallel analysis of macrophage extracts from Bcg(s) mouse strains (C57BL6/J and BALB/c, Nramp1(D169)) failed to detect mature Nrampl protein in these cells, suggesting that the G169D mutation at Nramp1 prevents proper maturation or membrane integration of the protein, resulting in its rapid degradation.