The yeast plasma membrane protein Alr1 controls Mg2+ homeostasis and is subject to Mg2+-dependent control of its synthesis and degradation

The Saccharomyces cerevisiae ALR1 (YOL130w) gene product Alr1p is the first known candidate for a Mg2+ transport system in eukaryotic cells and is dis tantly related to the bacterial CorA Mg2+ transporter family. Here we provi de the first experimental evidence for the location of Alr1p in the yeast p lasma membrane and for the tight control of its expression and turnover by Mg2+. Using well characterized npi1 and end3 mutants deficient in the endoc ytic pathway, we demonstrate that Alr1 protein turnover is dependent on ubi quitination and endocytosis. Furthermore, cells lacking the vacuolar protea se Pep4p accumulated Alr1p in the vacuole. Mutants lacking Alr1p (Delta alr 1) showed a 60% reduction of total intracellular Mg2+ compared with the wil d type and failed to grow in standard media. When starved of Mg2+, mutant a nd wild-type cells had similar low levels of intracellular Mg2+; but upon a ddition of Mg2+, wild-type cells replenished the intracellular Mg2+ pool wi thin a few hours, whereas Delta alr1 mutant cells did not. Expression of th e bacterial Mg2+ transporter CorA in the yeast Delta alr1 mutant partially restored growth in standard media. The results are discussed in terms of Al r1p being a plasma membrane transporter with high selectivity for Mg2+.