At the cellular level, the plasmatic membrane is a thin layer of lipids that surrounds the entire cell allowing the differentiation between the intracellular and extracellular environment. The physical barrier this lipid bilayer creates is dynamic and interactive, becoming the foundation for many interactions involved in GPCR signaling6. The cellular membrane's composition, organization, and physical properties might impact ligand binding, receptor activation, G protein interactions, signaling dynamics, and signal termination, highlighting the importance of studying their role in the activation of GPCRs2.
β-arrestins are cytosolic proteins that translocate to the plasma membrane upon GPCR activation, then regulate trigger receptor internalization via interaction with the adaptor protein 2 (AP2) and clathrin heavy chain mediating G protein-independent effects. At this point, the lipid bilayer serves as a platform for the membrane recruitment of β-arrestins. Lipid molecules, such as phosphoinositides, can bind to specific domains of β-arrestins, promoting their association with the plasma membrane. Therefore the lipid composition of the bilayer can influence the kinetics and efficiency of β-arrestin recruitment to the membrane, regulating their interactions with activated GPCRs2-6.
Understanding the interplay between GPCRs and β-arrestins and how this complex operates on the plasma membrane of living cells was the goal achieved by Jak Grimes et al. Based on an ingenious combination of multicolor single-molecule microscopy approach with molecular dynamics simulations, they dissected the sequence of events in receptor-β-arrestin interactions at the plasma membrane of living cells with ~20 nm spatial and ~30 ms temporal resolution. Whit this scope, the authors challenged the current model, wich suggest that β-arrestin translocates from the cytosol to bind an active receptor on the plasma membrane directly and remains attached to the same receptor until they reach clathrin-coated pits (CCPs)1.
However, the evidence obtained in this study proposes novel molecular mechanisms in which β-arrestin exhibits spontaneous pre-association with the plasma membrane. This pre-association enables β-arrestin to explore its surroundings through lateral diffusion and engage in highly transient interactions with receptors, ultimately leading to β-arrestin activation. This event extends the duration of β-arrestin at the plasma membrane, enabling it to independently reach clathrin-coated pits (CCPs) without solely relying on the initial and short-lived receptor-β-arrestin complexes 1.
Based on all the information collected, the authors proposed the following multistep model for receptor-β-arrestin interactions under unstimulated and stimulated conditions:
Unstimulated condition:
Inactive β-arrestin in the cytosol spontaneously binds to the plasma membrane by inserting the C-edge into the lipid bilayer, allowing it to explore space via lateral diffusion.
Most β-arrestin molecules remain on the plasma membrane briefly before dissociating and returning to the cytosol.
Stimulated condition in the presence of a stimulated receptor:
Spontaneous insertion into the plasma membrane β-arrestin.
β-arrestin reaches the receptor via lateral.
Transient interaction with the receptor catalyzes β-arrestin activation, including β-arrestin inter-domain rotation and extension of the finger loop.
Following dissociation from the receptor, the interaction of the extended finger loop with the lipid bilayer likely contributes to stabilizing β-arrestin in a membrane-bound, active-like conformation.
The above causes β-arrestin molecules to stay longer and accumulate on the plasma membrane, allowing them to reach CCPs vial lateral diffusion separately from the activating receptors.
The increase in active β-arrestin molecules and the time they spend diffusing on the plasma membrane leads to their recruitment and accumulation in CCPs via interaction with AP2 and clathrin.
β-arrestin molecules tethered to CCPs bind receptors diffusing on the plasma membrane, also causing their recruitment and accumulation in CCPs.
Although this study has limitations, such as the absence of the flexible distal C-tail of βArr2 in the model used, its findings redefine the existing model of receptor-β-arrestin interactions. They shed light on the essential role of β-arrestin binding to the lipid bilayer for efficient interaction between β-arrestin and the receptor.
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References
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