MTSES decreased single-channel amplitude of cysless/I344C channels. (Top trace) A continuous recoding from a patch containing hundreds of channels first phosphorylated with PKA, ATP, and DTT (see Materials and methods) until the current is steady. The current in the presence of ATP was dramatically reduced by the treatment of MTSES. (Bottom traces) Expansions of segments I–III marked in the top trace. Segment I shows spontaneous openings in the absence of ATP. Segments II and III show the current relaxation after the removal of ATP.