Inwardly rectifying potassium (Kir) channels are gated by the interaction of their cytoplasmic regions with membrane-bound phosphatidylinositol-4,5-bisphosphate (PIP₂). In the present study, we examined how PIP₂ interaction regulates channel availability and channel openings to various subconductance levels (sublevels) as well as the fully open state in the strong inward rectifier Kir2.1 channel. Various Kir2.1 channel constructs were expressed in Xenopus oocytes and single channel or macroscopic currents were recorded from inside-out patches. The wild-type channel rarely visited the subconductance levels under control conditions. However, upon reducing Kir2.1 channel interaction with PIP₂ by a variety of interventions, including PIP2 antibodies, screening PIP₂ with neomycin, or mutating PIP₂ binding sites (e.g. K188Q), visitation to the sublevels was markedly increased before channels were converted to an unavailable mode in which they did not open. No channel activity was detected in channels with the double mutation K188A/R189A, a mutant which exhibits extremely weak interaction with PIP₂. By linking subunits together in tandem dimers or tetramers containing mixtures of WT and K188A/R189A subunits, we demonstrate that one functional PIP₂-interacting WT subunit is sufficient to convert channels from the unavailable to the available mode with a high open probability dominated by the fully open state, with similar kinetics as tetrameric WT channels. Occasional openings to sublevels become progressively less frequent as the number of WT subunits increases. Quantitative analysis reveals that the PIP₂Ì interaction with WT subunits exert strong positive cooperativity in both converting the channels from the unavailable to the available mode, and in promoting the fully open state over sublevels. We conclude that the PIP₂Ì interaction with only one Kir2.1 subunit is sufficient for the channel to become available and to open to its full conductance state. Interaction with additional subunits exerts positive cooperativity at multiple levels to further enhance channel availability and promote the fully open state.