Extracellular Mg super(2+) regulates intracellular Mg super(2+) and its subcellular compartmentation in fission yeast, Schizosaccharomyces pombe

Effects of extracellular magnesium ions ([Mg super(2+)] sub(o)) on intracellular free Mg super(2+) ([Mg super(2+)] sub(i)) and its subcellular distribution in single fission yeast cells, Schizosaccharomyces pombe, were studied with digital-imaging microscopy and an Mg super(2+) fluorescent probe (mag-fura-2). Using 0.44 mM [Mg super(2+)] sub(o), [Mg super(2+)] sub(i) in yeast cells was 0.91 plus or minus 0.08 mM. Elevation of [Mg super(2+)] sub(o) to 1.97 mM induced rapid (within 5 min) increments in [Mg super(2+)] sub(i) (2.18 plus or minus 0.11 mM). Lowering [Mg super(2+)] sub(o) to 0.06 mM, however, exerted no significant effects on [Mg super(2+)] sub(i) (0.93 plus or minus 0.14 mM), at least for periods of up to 30 min. Irrespective of the [Mg super(2+)] sub(o) used, the subcellular distribution of [Mg super(2+)] sub(i) remained heterogeneous, i.e. where the sub-plasma membrane region > cytoplasm > nucleus. [Mg super(2+)] in all three subcellular compartments increased significantly, two- to threefold, concomitant with [Mg super(2+)] sub(i) when placed in 1.97 mM [Mg super(2+)] sub(o). We conclude that [Mg super(2+)] sub(i) in fission yeast is maintained at a physiologic level when [Mg super(2+)] sub(o) is low, but intracellular free Mg super(2+) rapidly rises when [Mg super(2+)] sub(o) is elevated. Like most eukaryotic cells, yeast may have a Mg super(2+) transport system(s) which functions to maintain gradients of Mg super(2+) from the outside to inside the cell and among its subcellular compartments.