In summary, evidence obtained with a wide range of different methods is accumulating, which suggest that GPCRs function as dimers or oligomers. Although individual evidences can be, and most of them were, challenged based on methodical grounds, the multitude of available positive data suggest that some kind of dimerization, oligomerization, or clustering of most GPCRs does occur. However, it is apparently very difficult to obtain decisive evidence, whether these receptors are organized into well-defined structures, such as dimers or oligomers, and interact directly; or function in larger molecular complexes, where propagation of the information occurs via complex molecular networks.
It is likely that this issue will not be fully resolved until the necessary tools are developed to obtain high-resolution snapshots of GPCRs in their native environment. Although it may take a lot more time for structural biologists to achieve this goal, dimerization, or oligomerization of GPCRs is a very useful paradigm for pharmacologists to study properties of receptors, which require functionally important clustering, such as allosteric modulation of ligand binding, co-internalization, altered signaling properties, or cross-inhibition.
An elucidation of these interactions is an important immediate task since these are critical to understand the pharmacological effects of drugs targeted to receptors and to elucidate the physiological mechanism of action of hormones and other mediators that target GPCRs. The authors declare that there is no conflict of interest that would prejudice the impartiality of the present work.
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Oligomerization of a G protein-coupled receptor in neurons controlled by its structural dynamics
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Opioid Receptor Oligomerization
Vice versa, active receptor dimer configurations will likely require a conformationally flexible TM6 helix, i. Such a dimer-specific activity and functional selectivity could offer new opportunities to target GPCR function with medical drugs Hipser et al. Furthermore, an increasing number of studies suggests that dimerization and signaling of GPCRs are modulated by the surrounding membrane. This review will focus on how different types of lipids and other membrane components may influence dimerization patterns of GPCRs and thereby possibly regulate function and signaling.
There are two possible paths for how lipids may influence GPCRs association: by direct binding to the receptor surface, or indirectly by modulating the properties of the surrounding membrane. Here, we review and discuss available information for both mechanisms, as well as commonly employed methods for the study of GPCR oligomerization. Two frequently used methods to analyze GPCR dimer- or oligomerization are resonance energy transfer RET techniques and computational approaches such as molecular dynamics MD simulations.
The fluorescent proteins are usually variants of the green fluorescent protein GFP Shimomura et al. Due to the fact that the fluorophor proteins consist of approximately amino acids, hence are of comparable size as GPCRs themselves, the receptor stability needs to be tested and possible fluorophor-driven associations between the proteins should be excluded see e.
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As an alternative to the large GFP, small peptides binding extracellular fluorescein were presented that may in many cases reduce steric side effects Griffin et al. If the donor is excited via light waves at its absorbance wavelength and the acceptor is in close vicinity, FRET occurs between the donor and the acceptor resulting in a detectable emission of light from the acceptor see Figure 1. However, not only the distance but as well the relative orientations between donor and acceptor affect the energy transfer: If the dipoles are oriented perpendicular to each other, no FRET occurs.
Only a parallel orientation between donor and acceptor dipoles allows for a resonance energy transfer. Therefore, experimental instruments need to be justified carefully and extensive mathematical analysis of the collected signals is required for reliable conclusions. Additionally, the absence of FRET is not equivalent to the absence of protein-protein interactions, the fluorophors might not be in a close enough proximity or their relative dipole orientation might not be properly aligned, i.
Furthermore, the surrounding environment can quench the fluorescence up to a degree that no FRET signals can be detected. On the other hand, the overexpression of fluorescence proteins or high protein concentrations can lead to FRET between non-interacting proteins in close vicinity. Figure 1. In BRET assays, a bioluminescent protein e. The basic advantages of BRET techniques are the reduced background signal as compared to FRET methods, since the excitation is biochemically triggered instead of light-induced. Additionally, BRET enables to perform kinetical measurements, given that the signals can be detected for up to 30 min Cottet et al.
A notable drawback of BRET strategies as compared to FRET techniques is that the substrate not only excites the bioluminescent proteins at the cell surface but possibly as well cell-interior proteins. FRET and BRET techniques are very powerful tools to investigate protein association, however the interpretation of RET efficiencies can be rather challenging because different efficiencies can result from either an increased or decreased number of receptor oligomers or from conformational changes in preexisting complexes see Figure 1.
MD simulations provide atomistic detail of biomolecular processes at high spatial and time resolution and proved extremely useful in the study of GPCR dynamics, GPCR oligomer formation and in the analysis of the influence of the membrane environment on oligomerization Sabbadin et al. In general, MD simulations at atomistic resolution of biomolecular systems are limited to the timescale of hundreds of nanoseconds to a few microseconds.
However, protein aggregation occurs on timescales of tens of microseconds. Additionally, membrane systems typically contain more than , atoms and are therefore computationally rather expensive. A significant speedup is gained by switching to more coarse-grained resolutions.
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For example, in the widely used Martini coarse-grained CG model, four heavy atoms are grouped together in one superatom or bead that reflects the properties of the 4 atoms Marrink et al. In this way, the number of degrees of freedom is reduced, high frequency motions of hydrogen atoms are excluded and the diffusion of proteins is increased by a reduced friction between CG beads as compared to atoms. Taken together, the computation time required to study a biomolecular process at CG resolution is by — times reduced as compared to a corresponding atomistic simulation.
Figure 2. Coarse-grained MD simulations.
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A Several molecules are shown as overlays between the atomistic and the corresponding Martini coarse-grained models. B A typical coarse-grained simulation system as used in receptor dimerization is shown before and after the simulation Pluhackova et al. Of course this loss of resolution comes with several drawbacks as compared to atomistic simulations. One important aspect in this regard is the inability of the CG force field to describe conformational changes of proteins, which is due to the simple design of the protein backbone in the Martini force field Marrink and Tieleman, In fact, the secondary structure is constrained by an additional force network to keep a preset structure de Jong et al.
Internalization Dissociates β2-Adrenergic Receptors
In addition, the reduction of resolution also affects the energy landscape of molecular systems: free solvation energies can be reproduced quite accurately in CG systems as compared to atomistic simulations, however, the decomposition of these free energies into enthalpic and entropic contributions can differ strongly Marrink and Tieleman, The reduction of the number of degrees of freedom influences the entropy of the system.
In order to reproduce the correct free energy, the enthalpic energies are therefore increased in the Martini model. In the same context, electrostatic interactions between charged particles are described implicitly in order to compensate the reduced number of partial charges and dipoles as compared to atomistic force fields. Consequently, Coulombic interactions in apolar environments are too weak in CG Martini models Marrink and Tieleman, Additionally, interactions between proteins in aqueous solution Stark et al. As a cure, it was shown for the older variant of the Martini force field that a reduction of the Lennard-Jones interactions between protein beads may compensate this overestimation Stark et al.
Nonetheless, the Martini forcefield shows good performance for membrane systems, hence it is commonly used to investigate transmembrane proteins on long timescales Pluhackova et al.