EVIDENCE FOR PROTEIN PHOSPHATASE-1 AND PHOSPHATASE-2A REGULATION OF K-CELLS( CHANNELS IN 2 TYPES OF LEAF)

ton channels control ion fluxes across membranes, membrane potential, and signal transduction between and within cells. Protein kinases and phosphatases are important regulators involved in stimulus-response co upling in eukaryotic organisms. We have identified in extracts of Vici a faba leaf cells protein phosphatase activities inhibited by okadaic acid (OA) and calyculin A (CA), two inhibitors of protein phosphatases 1 and 2A. Using whole-cell patch-clamp techniques, we have demonstrat ed that inward K+ currents in guard cells are inhibited by nanomolar c oncentrations of OA or CA, whereas outward K+ currents are not affecte d. However, the same inhibitors enhance the magnitude of outward K+ cu rrents in mesophyll cells. A phosphatase antagonist, adenosine-5'-O-(3 -thiotriphosphate), has an effect similar to OA and CA on outward K+ c urrents in mesophyll cells. Our findings suggest that protein phosphat ases 1 and/or 2A play different physiological roles in modulating the activity of K+ channels in mesophyll cells and guard cells.