EXTRACELLULAR MG2-POMBE( REGULATES INTRACELLULAR MG2+ AND ITS SUBCELLULAR COMPARTMENTATION IN FISSION YEAST, SCHIZOSACCHAROMYCES)

Effects of extracellular magnesium ions ([Mg2+](o)) on intracellular f ree Mg2+ ([Mg2+](i)) and its subcellular distribution in single fissio n yeast cells, Schizosaccharomyces pombe, were studied with digital-im aging microscopy and an Mg2+ fluorescent probe (mag-fura-2). Using 0.4 4 mM [Mg2+](o) [Mg2+](i) in yeast cells was 0.91 +/- 0.08 mM. Elevatio n of [Mg2+](o) to 1.97 mM induced rapid (within 5 min) increments in [ Mg2+](i) (2.18 +/- 0.11 mM). Lowering [Mg2+](o) to 0.06 mM, however, e xerted no significant effects on [Mg2+](i) (0.93 +/- 0.14 mM), at leas t for periods of up to 30 min. Irrespective of the [Mg2+](o) used, the subcellular distribution of [Mg2+](i) remained heterogeneous, i.e. wh ere the sub-plasma membrane region > cytoplasm > nucleus. [Mg2+] in al l three subcellular compartments increased significantly, two- to thre efold, concomitant with [Mg2+](i) when placed in 1.97 mM [Mg2+](o). We conclude that [Mg2+](i) in fission yeast is maintained at a physiolog ic level when [Mg2+](o) is low, but intracellular free Mg2+ rapidly ri ses when [Mg2+](o) is elevated. Like most eukaryotic cells, yeast may have a Mg2+ transport system(s) which functions to maintain gradients of Mg2+ from the outside to inside the cell and among its subcellular compartments.