INTERCONVERSION OF FAST AND SLOW GATING MODES OF GCAC1, A GUARD-CELL ANION CHANNEL

For guard-cell protoplasts of Vicia faba L. we elucidated whether the slow (S-type) and rapid (R-type) activating anion channels represent d ifferent gating modes of GCAC1, the Guard Cell Anion Channel. in the w hole-cell configuration of the patch-clamp technique, GCAC1 was activa ted in a Ca2+- and nucleotide-dependent manner. Cell-free outside-out membrane patches were isolated to determine the relative contribution of different gating modes or channel types to the overall anion curren t in this cell type. Within 2-15 min after the loss of cytoplasmic con trol through patch excision, depolarization-activated 38-pS channels c haracterized by flickering openings in the millisecond range convert i nto a channel of similar conductance but prolonged open times (hundred s of milliseconds) and lack of pronounced voltage-dependence. The rapi d (R-type) and slow (S-type) gating modes exhibited similar ion select ivity but different susceptibility towards block by the stilbene deriv ative DNDS (4,4'-dinitrostilbene-2,2'-disulfonic acid). On R-type chan nels DNDS caused a flickering block and a shift in the threshold poten tial of activation whereas S-type channels remained unaffected. Becaus e of the striking similarities of both channels with respect to single -channel conductance and relative permeability sequence on the one han d, but time-dependent conversion of R- into S-type gating after patch- excision on the other, we conclude that the mode of action of GCAC1 is under the control of cytoplasmic factors.