Expression, purification, and electrophysiological characterization of a recombinant, fluorescent Kir6.2 in mammalian cells

The inwardly rectifying K+ (Kir) channel, Kir6.2, plays critical roles in physiological processes in the brain, heart, and pancreas. Although Kir6.2 has been extensively studied in numerous expression systems, a comprehensive description of an expression and purification protocol has not been reported. We expressed and characterized a recombinant Kir6.2, with an N-terminal decahistidine tag, enhanced green fluorescent protein (eGFP) and deletion of C-terminal 26 amino acids, in succession, denoted eGFP-Kir6.2Δ26. eGFP-Kir6.2Δ26 was expressed in HEK293 cells and a purification protocol developed. Electrophysiological characterization showed that eGFP-Kir6.2Δ26 retains native single channel conductance (64 ± 3.3 pS), mean open times (τ1 = 0.72 ms, τ2 = 15.3 ms) and ATP affinity (IC50 = 115 ± 25 μM) when expressed in HEK293 cells. Detergent screening using size exclusion chromatography (SEC) identified Fos-choline-14 (FC-14) as the most suitable surfactant for protein solubilization, as evidenced by maintenance of the native tetrameric structure in SDS-PAGE and western blot analysis. A two-step scheme using Co2+-metal affinity chromatography and SEC was implemented for purification. Purified protein activity was assessed by reconstituting eGFP-Kir6.2Δ26 in black lipid membranes (BLMs) composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (POPG), l-α-phosphatidylinositol-4,5-bisphosphate (PIP2) in a 89.5:10:0.5 mol ratio. Reconstituted eGFP-Kir6.2Δ26 displayed similar single channel conductance (61.8 ± 0.54 pS) compared to eGFP-Kir6.2Δ26 expressed in HEK293 membranes; however, channel mean open times increased (τ1 = 7.9 ms, τ2 = 61.9 ms) and ATP inhibition was significantly reduced for eGFP-Kir6.2Δ26 reconstituted into BLMs (IC50 = 3.14 ± 0.4 mM). Overall, this protocol should be foundational for the production of purified Kir6.2 for future structural and biochemical studies.