Resonance energy transfer between guanine nucleotide binding protein subunits and membrane lipids

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Abstract
Resonance energy transfer was used to estimate the distances of closest approach between fluorescent labels on G protein subunits (alpha, beta, and gamma) and the phospholipid bilayer surface. Fluorescein-labeled alpha, beta, and gamma subunits were the energy transfer donors and hexadecylaminoeosin (HAE) in phospholipid vesicles was the acceptor. Bovine brain G protein (alpha o, beta, and gamma subunits) were individually labeled with fluorescein as described in the accompanying paper [Kwon et al. (1993) Biochemistry (preceding paper in this issue)]. Fluorescein-labeled subunits were combined with the appropriate unlabeled G protein subunits and reconstituted into phospholipid vesicles. HAE quenched the fluorescein emission in a concentration-dependent manner which was dependent upon the G protein subunit labeled (gamma = beta > alpha o). From steady-state quenching data with hexadecylaminofluorescein (HAF) as a standard to determine the density of HAE in the phospholipid bilayer, the calculated distances between fluorescein-alpha o, -beta, and -gamma and HAE are 46 +/- 2, 38 +/- 3, and 37 +/- 2 A, respectively. Energy transfer was identified as the means of fluorescence quenching by two methods: (1) addition of 0.1% Lubrol reversed the quenching of fluorescein-labeled G protein subunits by HAE and (2) sensitized HAE emission was observed in the presence of fluorescein-labeled G protein subunits. These results are the first physical measurements of the distances between sites on G protein subunits and the lipid bilayer. These data demonstrate that the label in the beta and gamma subunits is closer to the lipid bilayer than that in the alpha subunit.