Kir1.1 channel regulates membrane potential and K؉ secretion in renal tubular cells. This channel is gated by intracellular protons, in which a lysine residue (Lys 80 ) plays a critical role. Mutation of the Lys 80 to a methionine (K80M) disrupts pH-dependent channel gating. To understand how an individual subunit in a tetrameric channel is involved in pH-dependent channel gating, we performed these studies by introducing K80M-disrupted subunits to tandem tetrameric channels. The pH sensitivity was studied in whole-cell voltage clamp and inside-out patches. Homomeric tetramers of the wild-type (wt) and K80M-disrupted channels showed a pH sensitivity almost identical to that of their monomeric counterparts. In heteromeric tetramers and dimers, pH sensitivity was a function of the number of wt subunits. Recruitment of the first single wt subunit shifts the pK a greatly, whereas additions of any extra wt subunit had smaller effects. Single-channel analysis revealed that the tetrameric channel with two or more wt subunits showed one substate conductance at ϳ40% of the full conductance, suggesting that four subunits act as two pairs. However, three and four substates of conductance were seen in the tetrameric wt-3K80M and 4K80M channels. Acidic pH increased long-time closures when there were two or more wt subunits. Disruption of more than two subunits led to flicking activity with appearance of a new opening event and loss of the long period of closures. Interestingly, the channel with two wt subunits at diagonal and adjacent configurations showed the same pH sensitivity, substate conductance, and long-time closure. These results thus suggest that one functional subunit is sufficient to act in the pH-dependent gating of the Kir1.1 channel, the channel sensitivity to pH increases with additional subunits, the full pH sensitivity requires contributions of all four subunits, and two subunits may be coordinated in functional dimers of either trans or cis configuration.Kir1.1 (ROMK1) is a member of the inward rectifier K ϩ (Kir) channel family. A functional channel consists of four Kir subunits, and each Kir subunit has two membrane-spanning helices and a pore-forming sequence (1, 2). The Kir1.1 channel was originally cloned from the outer medulla of the kidney and later found in several other tissues (3, 4). The channel regulates K ϩ recycling in the thick ascending limb of Henle and K ϩ secretion in the cortical collecting duct (4, 5). Dysfunction of the Kir1.1 channel is one of the major causes for Bartter syndrome, which is characterized by decreased salt transport in the thick ascending limb of Henle (5). Kir1.1 surface expression and trafficking rely on protein kinase A and protein kinase C (6, 7). Activity of the Kir1.1 channel is directly regulated by intracellular pH, protein kinase A, protein kinase C, phosphatidylinositol-4,5-biphosphate, and WNK4 (serine-threonine kinase 4 with no lysine) (8 -11). Inhibition of protein kinase A activity or phosphatidylinositol-4,5-biphosphate production shifts the channel sen...