GIRK1 and GIRK4 subunits combine to form the heterotetrameric acetylcholine-activated potassium current (IKACh) channel in pacemaker cells of the heart. The channel is activated by direct binding of G-protein Gbetagamma subunits. The GIRK1 subunit is atypical in the GIRK family in having a unique ( approximately 125-amino acid) domain in its distal C terminus. GIRK1 cannot form functional channels by itself but must combine with another GIRK family member (GIRK2, GIRK3, or GIRK4), which are themselves capable of forming functional homotetramers. Here we show, using an extracellularly Flag-tagged GIRK1 subunit, that GIRK1 requires association with GIRK4 for cell surface localization. Furthermore, GIRK1 homomultimers reside in core-glycosylated and nonglycosylated states. Coexpression of GIRK4 caused the appearance of the mature glycosylated form of GIRK1. [35S]Methionine pulse-labeling experiments demonstrated that GIRK4 associates with GIRK1 either during or shortly after subunit synthesis. Mutant and chimeric channel subunits were utilized to identify domains responsible for GIRK1 localization. Truncation of the unique C-terminal domain of Delta374-501 resulted in an intracellular GIRK1 subunit that produced normal IKACh-like channels when coexpressed with GIRK4. Chimeras containing the C-terminal domain of GIRK1 from amino acid 194 to 501 were intracellularly localized, whereas chimeras containing the C terminus of GIRK4 localized to the cell surface. Deletion analysis of the GIRK4 C terminus identified a 25-amino acid region required for cell surface targeting of GIRK1/GIRK4 heterotetramers and a 25-amino acid region required for cell surface localization of GIRK4 homotetramers. GIRK1 appeared intracellular in atrial myocytes isolated from GIRK4 knockout mice and was not maturely glycosylated, supporting an essential role for GIRK4 in the processing and cell surface localization of IKACh in vivo.