FRET-based analysis of TRPC subunit stoichiometry

By analogy to other cation channel subunits with six transmembrane-spanning domains, the seven members of the "classical" or "canonical" transient receptor potential channels (TRPC) family are believed to assemble into homo- or heterotetrameric complexes. These complexes have been verified by classical methods such as coimmunoprecipitation, crosslinking analysis or functional assays applying dominant negative pore mutants. More recently, fluorescence resonance energy transfer (FRET)-a measure for the close proximity of fluorescent molecules-has become instrumental in monitoring protein assembly in living cells. Here we demonstrate further possibilities and verification procedures of the FRET technology to test the assembly of ion channel subunits. Temporally and spatially resolved FRET imaging demonstrates an early assembly of TRPC subunits in the endoplasmic reticulum and the Golgi apparatus. Confocal FRET imaging verifies FRET signals over the plasma membrane at high spatial resolution. Taking advantage of the quantitative analysis of digital video imaging, we demonstrate that FRET between TRPC subunits is only poorly concentration-dependent. Moreover, a correlation between the efficiency of energy transfer and the molar ratio of the FRET donor to the acceptor was exploited to verify the tetrameric stoichiometry of TRPC complexes. Finally, we introduce a competition-FRET assay to test the ability of wild-type TRPC subunits to recruit fluorescent TRPC subunits into separate channel complexes.