ATP-sensitive K+ channels are an octameric assembly of two proteins, a sulfonylurea receptor (SUR1) and an ion conducting subunit (Kir 6.0). We have examined the role of the C-terminus of SUR1 by expressing a series of truncation mutants together with Kir6.2 stably in HEK293 cells. Biochemical analyses using coimmunoprecipitation indicate that SUR1 deletion mutants and Kir6.2 assemble and that a SUR1 deletion mutant binds glibenclamide with high affinity. Electrophysiological recordings indicate that ATP sensitivity is normal but the response of the mutant channel complexes to tolbutamide, MgADP and diazoxide is disturbed. Quantitative immunofluorescence and cell surface biotinylation supports the idea that there is little disturbance in the efficiency of trafficking. Our data show that deletions of the C-terminal most cytoplasmic domain of SUR1, can result in functional channels at the plasma membrane in mammalian cells that have an abnormal response to physiological and pharmacological agents.