Reconstitution of purified membrane protein dimers in lipid nanodiscs with defined stoichiometry and orientation...

Many membrane proteins function as dimers or larger oligomers, including transporters, channels, certain signaling receptors, and adhesion molecules. In some cases, the interactions between individual proteins may be weak and/or dependent on specific lipids, such that detergent solubilization used for biochemical and structural studies disrupts functional oligomerization. Solubilized membrane protein oligomers can be captured in lipid nanodiscs, but this is an inefficient process that can produce stoichiometrically and topologically heterogeneous preparations. Here, we describe a technique to obtain purified homogeneous membrane protein dimers in nanodiscs using a split GFP (sGFP) tether. Complementary sGFP tags associate to tether the coexpressed dimers and control both stoichiometry and orientation within the nanodiscs, as assessed by quantitative Western blotting and negative-stain EM. The sGFP tether confers several advantages over other methods: it is highly stable in solution and in SDS-PAGE, which facilitates screening of dimer expression and purification by fluorescence, and also provides a dimer-specific purification handle for use with GFP nanobody-conjugated resin. We used this method to purify a Frizzled-4 homodimer and a Frizzled-4/low-density lipoprotein receptor-related protein 6 heterodimer in nanodiscs. These examples demonstrate the utility and flexibility of this method, which enables subsequent mechanistic molecular and structural studies of membrane protein pairs.