A MEMBRANE-DELIMITED PATHWAY OF G-PROTEIN REGULATION OF THE GUARD-CELL INWARD K+ CHANNEL

GTP-binding protein (G-protein) regulation of inward rectifying K+ cha nnels in the plasma membrane of Vicia (Vicia faba L.) guard cells has previously been demonstrated at the whole cell level. However, whether a cytosolic signal transduction chain is required for G-protein regul ation of K+ channels in Vicia guard cells, or in any plant cell type, remains unknown. In the present study, we assayed effects of several G -protein regulators on inward K+ channels in isolated inside out membr ane patches from Vicia guard cell protoplasts. Guanosine 5'-[gamma-thi o]triphosphate, a nonhydrolyzable GTP analog that locks G proteins int o their activated state, decreased the open state probability (P-o) of single inward K+ channels. This decrease in P-o was accompanied by an increase in one of the closed time constants of the K+ channel. Guano sine 5'- [beta-thio]diphosphate, a GDP analog that locks G proteins in to their inactivated state, slightly increased the P-o of the inward K + channel and shortened the closed time constants. Pertussis toxin and cholera toxin, which ADP-ribosylate G proteins at different sites, de creased the P-o of the inward K+ channel. Our data indicate that G pro teins can act via a membrane-delimited pathway to regulate inward K+ c hannels in the guard-cell plasma membrane.