Transducin Activation by Nanoscale Lipid Bilayers Containing One and Two Rhodopsins

Abstract

Nanodiscs are nanometer scale planar membranes of controlled size that are rendered soluble in aqueous solution via an encircling amphipathic membrane scaffold protein "belt" (Bayburt, T. H., Grinkova, Y. V., and Sligar, S. G. (2002) Nano. Lett. 2, 853-856). Integral membrane proteins can be self-assembled into the Nanodisc bilayer with defined stoichiometry, which allows an unprecedented opportunity to investigate the nature of the oligomerization state of a G-protein-coupled receptor and its coupling to heterotrimeric G-proteins. We generated Nanodiscs having one and two rhodopsins present in the 10-nmdiameter lipid bilayer domain. Efficient transducin activation and isolation of a high affinity transducin-metarhodopsin II complex was demonstrated for a monodisperse and monomeric receptor. A population of Nanodiscs containing two rhodopsins was generated using an increased ratio of receptor to membrane scaffold protein in the self-assembly mixture. The two-rhodopsin population was isolated and purified by density gradient centrifugation. Interestingly, in this case, only one of the two receptors present in the Nanodisc was able to form a stable metarhodopsin II-G-protein complex. Thus there is clear evidence that a monomeric rhodopsin is capable of full coupling to transducin. Importantly, presumably due to steric interactions, it appears that only a single receptor in the Nanodiscs containing two rhodopsins can interact with G-protein. These results have important implications for the stoichiometry of receptor-G-protein coupling and cross talk in signaling pathways.A growing pool of evidence suggests the facile homo-and heterodimerization of G-protein-coupled receptors (GPCRs) 2 with potential functional implications for cellular signaling. These conclusions are based largely on coimmunoprecipitation, resonance energy transfer between labeled GPCRs in cells, functional complementation, and in vitro analysis of isolated receptors (1). For example, purified leukotriene receptor BLT1 has been shown to exist as a dimer in detergent, which forms a pentameric complex of dimer with heterotrimeric G-protein and demonstrates cooperative interaction with ligand (2, 3). Oligomeric arrays of rhodopsin, a class A GPCR of the visual transduction system, have been observed in the atomic force microscope using native disk membranes (4 -6). Rhodopsin dimers have been observed in uranyl formate-stained preparations by electron microscopy (8) and have also been studied in detergent-solubilized preparations (9, 10). Recent evidence for self-association of rhodopsin in liposomes has been obtained by fluorescence techniques (11) consistent with earlier work (12). Other investigations, using detergents with varying properties, isolated and characterized rhodopsin in different oligomeric states, with the conclusion that oligomers activate transducin more efficiently (13). In contrast to these studies, rhodopsin has long been thought to function as a monomer based on extensive biophysical evidence, as reviewed in Ref. 14.To resolve t...

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“…In spite of the evidence that rhodopsin and the b 2 -adrenergic receptor (b2-AR) activate their cognate G protein in the monomeric form [9][10][11][12] and that supramolecular organizations of rhodopsin [9], neurotensin 1 receptor [13] and leukotriene B 4 receptor (BLT 2 ) [14] reduce G protein coupling, formation of GPCR oligomers in living cells has been widely demonstrated [15][16][17]. Indeed, it was recently inferred that the b 2 -AR exists in dynamic equilibrium between monomeric and higher-order oligomers, with the average size of the oligomer being a tetramer and with inverse agonists promoting higher order oligomerization [15].…”

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“…It is well known from in vitro and in vivo experiments that GPCR monomers can recognize/decode signals. In this respect, worth mentioning are studies in which the monomeric entities of three class A GPCRs (namely rhodopsin, β 2 -adrenergic, and μ-opioid receptors) trapped into nanodiscs were able to signal as monomers (Bayburt et al, 2007;Whorton et al, 2007;Kuszak et al, 2009). Furthermore, signaling from GPCR monomers is characterized by an intrinsic plasticity, as GPCR activation can result in different patterns of signal transduction, such as G protein and/or arrestin pathways (Zidar et al, 2009).…”