Cation permeability and selectivity of a root plasma membrane calcium channel

Calcium channels in the plasma membrane of root cells fulfill both nutritio nal and signaling roles, The permeability of these channels to different ca tions determines the magnitude of their cation conductances, their effects on cell membrane potential and their contribution to cation toxicities. The selectivity of the rca channel, a Ca2+-permeable channel from the plasma m embrane of wheat (Triticum aestivum L,) roots, was studied following its in corporation into planar lipid bilayers. The permeation of K+, Na+, Ca2+ and Mg2+ through the pore of thr: Ii-a channel was modeled. It was assumed tha t cations permeated in single file through a pore with three energy barrier s and two ion-binding sites. Differences in permeation between divalent and monovalent cations were attributed largely to the affinity of the ion bind ing sites. The model suggested that significant negative surface charge was present in the vestibules to the pore and that the pore could accommodate two cations simultaneously. which repelled each other strongly. The pore st ructure of the rca channel appeared to differ from that of L-type calcium c hannels from animal cell membranes since its ion binding sites had a lower affinity for divalent cations. The model adequately accounted for the diver se permeation phenomena observed for the rca channel. It described the appa rent submillimolar K-m for the relationship between unitary conductance and Ca2+ activity, the differences in selectivity sequences obtained from meas urements of conductance and permeability ratios, the changes in relative ca tion permeabilities with solution ionic composition, and the complex effect s of Ca2+ on K+ and Na+ currents through the channel. Having established th e adequacy of the model, it was used to predict the unitary currents that w ould be observed under the ionic conditions employed in patch-clamp experim ents and to demonstrate the high selectivity of the rca channel for Ca2+ in flux under physiological conditions.