Acts as a highly selective magnesium channel. The channel activity is regulated by the intracellular magnesium concentration. Under high-intracellular magnesium conditions, binding of magnesium to the cytosolic domains stabilizes the closed conformation of the channel. Under low-intracellular magnesium conditions, the channel is in equilibrium between the open and closed states. Homodimer. Belongs to the SLC41A transporter family.