FUNCTION OF METAL-ION HOMEOSTASIS IN THE CELL-DIVISION CYCLE, MITOCHONDRIAL PROTEIN PROCESSING, SENSITIVITY TO MYCOBACTERIAL INFECTION AND BRAIN-FUNCTION

A novel Saccharomyces cerevisiae mutant, unable to grow in the presenc e of 12.5 mmol l(-1) EGTA, was isolated, The phenotype of the mutant i s caused by a single amino acid change (Gly149 to Arg) in the essentia l yeast cell division cycle gene CDC1. The mutant could be suppressed by overexpression of the SMF1 gene, which codes for a plasma membrane Mn2+ transporter. We observed that the yeast SMF1 gene shares homology with the mouse Nramp gene. Nramp (Bcg) was cloned as a gene responsib le for mouse resistance to infection with mycobacteria and is identica l with the Ity and the Lsh genes conferring resistance to infection by Salmonella typhimurium and Leishmania donovani, respectively. Althoug h the cloning of Nramp identified the gene responsible for the resista nce of mice to mycobacteria, its function is unknown. We propose that the mammalian protein, like the yeast transporter, is a Mn2+ and/or Zn 2+ transporter. Following the phagocytosis of a parasite into the phag osome, the macrophage produces reactive oxygen and/or nitrogen interme diates that are toxic for the internalized bacteria, The survival of t he pathogen during the burst of macrophage respiratory activity is tho ught to be partly mediated by microbial superoxide dismutase (SOD), wh ich contains Mn2+ or Fe2+ in its active centre, Nramp may transport Mn -2+ from the extracellular milieu into the cytoplasm of a macrophage a nd, after the generation of the phagosome, remove Mn2+ from the organe lle. Thus, the Mn2+-depletion of the phagosome microenvironment by the Nramp gene product may be a rate-limiting step in the metalloenzyme's production by the engulfed bacteria, This limitation will restrict th e mycobacterial ability to produce active enzymes such as SOD and prev ent the propagation of the ingested microorganisms, Conversely, an inc reased concentration of Mn2+ in the phagosome caused by a defective Nr amp transporter (Bcg(S)) may promote the growth of the mycobacteria an d render the organism sensitive to the pathogen. We use a similar appr oach to identify, clone and study other metal-ion transporters.