Detergent-free solubilization of human Kv channels expressed in mammalian cells

Styrene-maleic acid (SMA) copolymers are used to extract lipid-encased membrane proteins from lipid bilayers in a detergent-free manner, yielding SMA lipid particles (SMALPs). SMALPs can serve as stable water-soluble nanocontainers for structural and functional studies of membrane proteins. Here, we used SMA copolymers to study full-length pore-forming α-subunits hKCNH5 and hKCNQ1 of human neuronal and cardiac voltage-gated potassium (Kv) channels, as well as the fusion construct comprising of an α-subunit hKCNQ1 and its regulatory transmembrane KCNE1 β-subunit (hKCNE1-hKCNQ1) with added affinity tags, expressed in mammalian COS-1 cells. All these recombinant proteins were shown to be functionally active. Treatment with the SMA copolymer, followed by purification on the affinity column, enabled extraction of all three channels. A DLS experiment demonstrated that negative stain electron microscopy and single particle image analysis revealed a four-fold symmetry within channel-containing SMALPs, which indicates that purified hKCNH5 and hKCNQ1 channels, as well as the hKCNE1-hKCNQ1 fusion construct, retained their structural integrity as tetramers.