2 PHYSICALLY DISTINCT PORES IN THE DIMERIC CIC-0 CHLORIDE CHANNEL

THE Torpedo chloride channel ClC-0 (ref, 1) is the prototype of a larg e family of chloride channels that have roles in transepithelial trans port(2) and in regulating electrical excitability(3-6) and cell volume (7). ClC-0 opens in bursts with two identical conductance levels of si milar to 8 pS (refs 8-10), Hyperpolarization slowly increases the prob ability of bursts ('slow gating'), and depolarization increases channe l opening within bursts ('fast gating'). Replacing serine 123 by threo nine changes rectification, ion selectivity and gating, but retains th e typical bursting behaviour with two identical independent albeit red uced, conductance states (similar to 1.5 pS), Coexpression with wild-t ype ClC-0, either as covalently linked concatamers or as independent p roteins, leads to bursting channels with two different pores. Our expe riments strongly suggest that conductance, ion selectivity and 'fast' gating are determined only by the single subunit forming a single pore , independent from the attached pore; in contrast, 'slow' gating is a function of both subunits. Thus ClC-0 is a homodimer with two largely independent pores.