Molecular and cellular mechanisms underlying iron transport deficiency in microcytic anemia.

A mutation of the iron transporter Nramp2 (DMT1, Slc11a2) causes microcytic anemia in mk mice and in Belgrade rats by impairing iron absorption in the duodenum and in erythroid cells, causing severe iron deficiency. Both mk and Belgrade animals display a glycine-to-arginine substitution at position 185 (G185R) in the fourth predicted transmembrane domain of Nramp2. To study the molecular basis for the loss of function of Nramp2(G185R), we established cell lines stably expressing extracellularly tagged versions of wild-type (WT) or mutated transporters. Like WT Nramp2, the G185R mutant was able to reach the plasmalemma and endosomal compartments, but with reduced efficiency. Instead, a large fraction of Nramp2(G185R) was detected in the endoplasmic reticulum, where it was unstable and was rapidly degraded by a proteasome-dependent mechanism. Moreover, the stability of the mutant protein that reached the plasma membrane was greatly reduced, further diminishing its surface density at steady state. Last, the specific metal transport activity of plasmalemmal Nramp2(G185R) was found to be significantly depressed, compared with its WT counterpart. Thus, a singlepoint mutation results in multiple biosynthetic and functional defects that combine to produce the impaired iron deficiency that results in microcytic anemia.