The ROMK1 (Kir 1.1a) channel is formed by a tetrameric complex of subunits, each characterized by cytoplasmic N- and C-termini and a core region of two transmembrane helices flanking a pore-forming segment. To delineate the general regions mediating the assembly of ROMK1 subunits we constructed epitope-tagged N-terminal, C-terminal, and transmembrane segment deletion mutants. Nonfunctional subunits with N-terminal, core region, and C-terminal deletions had dominant negative effects when coexpressed with wild-type ROMK1 subunits in Xenopus oocytes. In contrast, coexpression of these nonfunctional subunits with Kv 2.1 (DRK1) did not suppress Kv 2.1 currents in control oocytes. Interactions between epitope-tagged mutant and wild-type ROMK1 subunits were studied in parallel by immunoprecipitating [35S]-labeled oocyte membrane proteins. Complexes containing both wild-type and mutant subunits that retained H5, M2, and C-terminal regions were coimmunoprecipitated to a greater extent than complexes consisting of wild-type and mutant subunits with core region and/or C-terminal deletions. The present findings are consistent with the hypothesis that multiple interaction sites located in the core region and cytoplasmic termini of ROMK1 subunits mediate homomultimeric assembly.