Alr1p is an integral plasma membrane protein essential for uptake of Mg(2+) into yeast cells. Homologs of Alr1p are restricted to fungi and some protozoa. Alr1-type proteins are distant relatives of the mitochondrial and bacterial Mg(2+)-transport proteins, Mrs2p and CorA, respectively, with which they have two adjacent TM domains and a short Mg(2+) signature motif in common. The yeast genome encodes a close homolog of Alr1p, named Alr2p. Both proteins are shown here to be present in the plasma membrane. Alr2p contributes poorly to Mg(2+) uptake. Substitution of a single arginine with a glutamic acid residue in the loop connecting the two TM domains at the cell surface greatly improves its function. Both proteins are shown to form homo-oligomers as well as hetero-oligomers. Wild-type Alr2p and mutant Alr1 proteins can have dominant-negative effects on wild-type Alr1p activity, presumably through oligomerization of low-function with full-function proteins. Chemical cross-linking indicates the presence of Alr1 oligomers, and split-ubiquitin assays reveal Alr1p-Alr1p, Alr2p-Alr2p, and Alr1p-Alr2p interactions. These assays also show that both the N-terminus and C-terminus of Alr1p and Alr2p are exposed to the inner side of the plasma membrane.