The minimal K+ channel protein (minK, also called ISK) is structurally dissimilar to other cloned voltage-gated ion channels. minK is a 15-kD polypeptide with only one potential transmembrane helix. Published data suggest that the current associated with minK expression in Xenopus oocytes may be related to the slow cardiac delayed rectifier K+ current (IKs). However, the fact that minK expression has been limited exclusively to Xenopus oocytes has caused continuing concern about the nature of this protein and its molecular link to known mammalian K+ channels. We report in the present study the first expression of minK activity in transiently transfected mammalian (HEK 293) cells and demonstrate that the characteristics of the expressed minK current are similar to those of IKs recorded from guinea pig heart cells under similar experimental conditions. We also show that an antibody directed against the minK channel protein reacts with a surface antigen on adult guinea pig ventricular myocytes and sinoatrial nodal cells, where IKS is the dominant outward K+ current. The data provide strong evidence that a minK-like protein underlies IKs.