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 distantly related to the bacterial CorA Mg2+ transporter family. Here we provide the first experimental evidence for the location of Alr1p in the yeast plasma membrane and for the tight control of its expression and turnover by Mg2+. Using well characterized npi1 and end3 mutants deficient in the endocytic pathway, we demonstrate that Alr1 protein turnover is dependent on ubiquitination and endocytosis. Furthermore, cells lacking the vacuolar protease Pep4p accumulated Alr1p in the vacuole. Mutants lacking Alr1p (Delta alr1) showed a 60% reduction of total intracellular Mg2+ compared with the wild type and failed to grow in standard media. When starved of Mg2+, mutant and wild-type cells had similar low levels of intracellular Mg2+; but upon addition of Mg2+, wild-type cells replenished the intracellular Mg2+ pool within a few hours, whereas Delta alr1 mutant cells did not. Expression of the bacterial Mg2+ transporter CorA in the yeast Delta alr1 mutant partially restored growth in standard media. The results are discussed in terms of Alr1p being a plasma membrane transporter with high selectivity for Mg2+.