Potassium-dependent changes in the expression of membrane-associated proteins in barley roots : I. Correlations with k(rb) influx and root k concentration

Abstract

Barley (Hordeum vulgare L. cv Halcyon) seedlings which had been grown in full strength complete inorganic nutrient media (containing 6 millimolar K(+)) had high internal K(+) concentrations and low values of K(+) ((86)Rb(+)) influx when influx was measured from solutions containing 100 micromolar K(+). Transfer of these plants to solutions lacking K(+) resulted in significant reductions of root and shoot K(+) concentrations and values of K(+) ((86)Rb(+)) influx increased by greater than 10-fold within 3 days. When plants treated in this way were returned to complete solutions, containing K(+), the changes induced by K(+) deprivation were reversed. Parallel studies of microsomal membranes by means of SDS-PAGE demonstrated that the expression of a group of polypeptides increased or decreased in parallel with changes of K(+) ((86)Rb(+)) influx. Most prominent of these were 45 and 34 kilodalton polypeptides which specifically responded to K(+) status of the barley plants; their expression was not enhanced by N or P deprivation. The 45 kilodalton polypeptide was susceptible to degradation by a membrane associated protease when microsomes were washed in buffer containing 0.2 millimolar PMSF. This loss was prevented by increasing PMSF concentration to 2 millimolar.