HUMAN NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN - CDNA CLONING, CHROMOSOMAL MAPPING, GENOMIC ORGANIZATION, AND TISSUE-SPECIFIC EXPRESSION

Natural resistance to infection with unrelated intracellular parasites such as Mycobacteria, Salmonella, and Leishmania is controlled in the mouse by a single gene on chromosome 1, designated Bcg, Ity, or Lsh. A candidate gene for Beg, designated natural resistance-associated mac rophage protein (Nramp), has been isolated and shown to encode a novel macrophage-specific membrane protein, which is altered in susceptible animals. We have cloned and characterized cDNA clones corresponding t o the human NRAMP gene. Nucleotide and predicted amino acid sequence a nalyses indicate that the human NRAMP polypeptide encodes a 550-amino acid residue membrane protein with 10-12 putative transmembrane domain s, two N-linked glycosylation sites, and an evolutionary conserved con sensus transport motif. Identification of genomic clones corresponding to human NRAMP indicates that the gene maps to chromosome 2q35 within a group of syntenic loci conserved with proximal mouse 1. The gene is composed of at least 15 exons, with several exons encoding discrete p redicted structural domains of the protein. These studies have also id entified an alternatively spliced exon encoded by an Alu element prese nt within intron 4. Although this novel exon was found expressed in vi vo, it would introduce a termination codon in the downstream exon V, r esulting in a severely truncated protein. Northern blot analyses indic ate that NRAMP mRNA expression is tightly controlled in a tissue-speci fic fashion, with the highest sites of expression being peripheral blo od leukocytes, lungs, and spleen. Additional RNA expression studies in cultured cells identified the macrophage as a site of expression of h uman NRAMP and indicated that increased expression was correlated with an advanced state of differentiation of this lineage.