1. Author Response:

    Reviewer #1 (Public Review):

    The physical principles underlying oligomerization of GPCRs are not well understood. Here, authors focused on oligomerization of A2AR. They found that oligomerization of A2AR is mediated by the intrinsically disordered, extramembraneous C-terminal tail. Using experiment and MD simulation, they mapped the regions that are responsible for oligomerization and dissected the driving forces in oligomerization.

    This is a nice piece of work that applies fundamental physical principles to the understanding of an important biological problem. It is a significant finding that oligomerization of A2AR is mediated by multiple weak interactions that are "tunable" by environmental factors. It is also interesting that solute-induced, solvent-mediated "depletion interactions" can be a key driving force in membrane protein-protein interactions.

    Although this work is potentially a significant contribution to the fields of GPCRs and molecular biophysics of membrane proteins in general, there are several concerns that would need to be implemented to strengthen the conclusions.

    1. How reasonably would the results obtained in the micellar environment be translated into the phenomenon in the cell membranes?

    1a) Here authors measured oligomerization of A2AR in detergent micelles, not in the bilayer or cellular context. Although the cell membranes would provide another layer of complexity, the hydrophobic properties and electrostatics of the negatively charged membrane surface may cooperate or compete with the interactions mediated by the C-terminal tail, especially if the oligomerization is mediated by multiple weak interactions.

    The translatability of properties of membrane proteins in detergent micelles to the cellular context is a valid concern. However, this shortcoming applies to all biophysical studies of membrane proteins in non-native environments. Even for membrane proteins reconstituted in liposomes, the question arises whether the artificial lipid composition that differs from that in the human plasma membrane would alter protein properties, especially as surface charges and cholesterol content can impact membrane protein dynamics, association, and stability. In that sense, this question cannot be answered satisfyingly, especially for GPCRs that are notoriously difficult to isolate. However, we can offer some perspectives. The propensity for membrane proteins to associate and oligomerize, if anything, is greater in lipid bilayers compared to that in detergent micelles, while detergent micelles can effectively solubilize membrane protein monomers (Popot and Engelman, Biochem 1990, 29 (17), 4031–4037). Hence, the findings that A2AR readily oligomerizes in detergent micelles and that the degree of oligomerization can be systematically tuned by the C-terminal length of A2AR in the same micellar system suggest that inter-A2AR interactions are modulating receptor oligomerization; we speculate that A2AR oligomers will be present or be enhanced in the lipid bilayer environment. In fact, in the cellular context, it has been shown that A2AR assembles into homodimers at the cell surface in transfected HEK293 cells (Canals et al, J Neurochem 2004, 88, 726–734) and into higher- order oligomers at the plasma membrane in Cath.A differentiated neuronal cells (Vidi et al, FEBS Lett 2008, 582, 3985–3990). Furthermore, C-terminally truncated A2AR has been demonstrated to show no protein aggregation or clustering on the cell surface, a process otherwise observed in the WT form (Burgueno et al, J Biol Chem 2003, 278 (39), 37545–37552). These results provide the research community with a valid starting point to discover factors that control oligomerization of A2AR in the cellular context.

    1b) Related to the point above (1a), I wonder if MD simulation could provide an insight into the role of the lipid bilayer in the inter- or intra-molecular interactions involving the tail. Although the neutral POPC bilayer was employed in the simulation, the tail-membrane interaction may affect oligomerization since the tail is intrinsically disordered and possess a significant portion of nonpolar residues (Fig. S4).

    The reviewer brings up a valid point about the ability for MD simulations to provide insights into the role of membrane-protein interactions. In response to the reviewer, we performed additional analysis focusing on the interactions of the C-terminus with the lipid bilayer. Overall, as the C-terminus is extended, there is a decrease in its interaction with the cytoplasmic leaflet of the membrane (left figure below). More specifically, we find that the C-terminal segment associated with helix 8 (residues 291 to 314) interacts tightly with the membrane, while the rest of the C-terminus (an intrinsically disordered segment) more weakly interacts with the membrane, regardless of truncation (right figure below). As the C-terminus is extended, the inherent conformational flexibility leads to a decrease in the interactions between the protein and the bilayer. We also observe that shorter stretches of the disordered segment do have the ability to interact more closely with the membrane. While these portions include charged residues that can participate in formation of the dimer interface, no general trends are observed. We therefore cannot draw any conclusions regarding the role of C-terminal-membrane interactions on the dimerization of A2AR. What we do know is that the MD simulations presented here should be considered a model study that reveals that the charged and disordered C-terminus of A2AR can account for oligomerization via multiple and weak inter-protomer contacts.

    MD simulations showing (Left) average distance of all C-terminal residues and (right) average per-residue distance from the cytoplasmic membrane of the lipid bilayer.

    1. Ensuring that the oligomer distributions are thermodynamic products.

    Since authors interpret the SEC results on the basis of thermodynamic concepts (driving forces, depletion interactions, etc.), it would be important to verify that the distribution of different oligomeric states is the outcome of the thermodynamic control. There is a possibility that the distribution is the outcome of the "kinetic trapping" during detergent solubilization.

    This is an important question. As we have shown in the manuscript, the A2AR dimer level was found to be reduced in the presence of TCEP (Figure 2B), suggesting that disulfide linkages have a role in facilitating A2AR oligomerization. However, disulfide cross-linking reaction cannot be the sole driving force of A2AR oligomerization because (1) a significant population of A2AR dimer remained resistant to TCEP (Figure 2B), (2) A2AR oligomer levels decreased progressively with the shortening of the C-terminus (Figure 3), and (3) A2AR oligomerization is driven by depletion interactions enhanced with increasing ionic strength (Figure 5).

    To answer whether A2AR oligomer is a thermodynamic or kinetic product, we tested the stability and reversibility of the A2AR monomer and dimer/oligomer population. We used SEC to separate these populations of both the A2AR-WT and A2AR-Q372ΔC variants, then performed a second round of SEC to observe their repopulation, if any. The results are summarized in the figure below, which we will include in the revised manuscript as Figure 5-figure supplement 1.

    We find that the SEC-separated monomers repopulate measurably into dimer/oligomer, with the total oligomer level after redistribution comparable with that of the initial samples for both A2AR WT (initial: 2.87; redistributed: 1.60) and A2AR-Q372ΔC (initial: 1.49; redistributed: 1.40) (Figure 5-figure supplement 1A). This observation indicates that A2AR oligomer is a thermodynamic product with a lower free energy compared with that of the monomer. This is consistent with the results we have shown in the manuscript that the oligomer levels of A2AR-WT are consistent (1.34–2.87; Table S1) and that A2AR oligomerization can be modulated with ionic strengths via depletion interactions (Figure 5).

    Figure S5. The dimer/oligomerization of A2AR is a thermodynamic process where the dimer and HMW oligomer once formed are kinetically trapped. (A) SEC chromatograms of the consecutive rounds of SEC performed on A2AR-WT and Q372ΔC. The first rounds of SEC are to separate the dimer/oligomer population and the monomer population, while the second rounds of SEC are performed on these SEC-separated populations to assess their stability and reversibility. The total oligomer level is expressed relative to the monomeric population in arbitrary units. (B) Energy diagram depicting A2AR oligomerization progress. The monomer needs to overcome an activation barrier (EA), driven by depletion interactions, to form the dimer/oligomer. Once formed, the dimer/oligomer populations are kinetically trapped by disulfide linkages.

    Interestingly, the SEC-separated dimer/oligomer populations do not repopulate to form monomers (Figure 5-figure supplement 1). This observation is, again, consistent with a published study of ours on A2AR dimers (Schonenbach et al, FEBS Lett 2016, 590, 3295–3306). This observation furthermore indicates that once the oligomers are formed, some are kinetically trapped and thus cannot redistribute into monomers.

    We believe that disulfide linkages are likely candidates that kinetically stabilize A2AR oligomers, as demonstrated by their redistribution into monomers only in the presence of a reducing agent (Figure 2B). Taken together, we suggest that A2AR oligomerization is a thermodynamic process (Figure 5-figure supplement 1B), with the monomer overcoming the activation energy (EA) by depletion interactions to repopulate into dimer/oligomer with a slightly lower free energy (given that we see a distribution between the two). Once formed, the redistributed dimer/oligomer populations can be kinetically stabilized by disulfide linkages.

    1. The claim that the C-terminal tail is engaged in "cooperative" interactions is too qualitative (p. 11 line 274, p.12 line 279 and p.18 line 426).

    This claim seems derived from Fig. 3b and Figs. 4b-c. However, the gradual decrease in the dimer level and the number of interactions may indicate that different parts in the C-terminal tail contribute to dimerization additively rather than cooperatively. The large decrease in the number of interactions may stem from the large decrease in the length (395 to 354). Probably, a more quantitative measure would be the number of interactions (H-bonds/salt bridges) normalized to the tail length upon successive truncation. Even in that case, the polar/charged residues would not be uniformly distributed along the primary sequence, making the quantitative argument of cooperativity challenging.

    The request to clarify our basis to refer to a cooperative interaction is well taken. Figure 4B and 4C show that the truncation of one part of the C-terminus (segment 335–394) leads to a reduction in contacts of a different part (segment 291–334) of A2AR. Therefore, we conclude that the binding interactions that occur in segment 291–334 are altered by the interactions exerted by the segment 335–394. This characteristic is consistent with allosteric interactions. We believe that characterizing these interactions as “cooperative” is possible but is not fully justified in this work. We also agree with the comment that quantifying the role and segments involved in contacts would be challenging. The manuscript has been amended to use the term “allosteric” in place of “cooperative”.

    1. On the compactness and conformation of the C-terminal tail:

    Although the C-terminal tail is known as "intrinsically disordered", the results seem to indicate that its conformation is rather compact (or collapsed) with a number of intra- and intermolecular polar interactions (Fig. 4) and buried nonpolar residues (Fig. 6), which are subject to depletion interactions (Fig. 5). This raises a question if the tail indeed "intrinsically disordered" as is known. Recent folding studies on IDPs (Riback et al. Science 2017, 358, 238-; Best, Curr Opin Struct Biol 2020, 60, 27-) suggest that IDPs are partially expanded or expanded rather than collapsed.

    We agree that our results seem to suggest that the conformation of the C-terminus could be partially compact. However, by stating that the C-terminus on average is an intrinsically disordered region (IDR), we do not exclude the possibility of partially structured regions, or greater compactness than that of an excluded volume polymer. IDR or IDP should refer to all proteins or protein regions that do not adopt a unique structure. By that standard, we know that the C-terminus of A2AR falls into that category according to our experiments and MD simulation, as well as the literature. In isolation, the majority the C-terminus is indeed an IDR, as has been demonstrated not only by simulations but also by experimental data. In fact, the C-terminus exhibits partial alpha-helical structure, and transiently populates beta-sheet conformations, depending on its state and buffer conditions (Piirainen et al, Biophys J 2015, 108 (4), 903–917). The literature studies suggest that A2AR’s C-terminus may adopt a greater level of compactness when interactions are formed between the C-terminus and the rest of the A2AR oligomer.

    Reviewer #2 (Public Review):

    The authors expressed A2A receptor as wild type and modified with truncations/mutations at the C-terminus. The receptor was solubilized in detergent solution, purified via a C-terminal deca-His tag and the fraction of ligand binding-competent receptor separated by an affinity column. Receptor oligomerization was studied by size exclusion chromatography on the purified receptor solubilized in a DDM/CHAPS/CHS detergent solution. It was observed that truncation greatly reduces the tendency of A2A to form dimers and oligomers. Mechanistic insights into interactions that facilitate oligomerization were obtained by molecular simulations and the study of aggregation behavior of peptide sequences representing the C-terminus of A2A. It is concluded that a multitude of interactions including disulfide linkages, hydrogen bonds electrostatic- and depletion interactions contribute to aggregation of the receptor.

    The general conclusions appear to be correct and the paper is well written. This is a study of protein association in detergent solution. It is conceivable that observations are relevant for A2A receptors in cell membranes as well. However, extrapolation of mechanisms observed on receptor in detergent micelles to receptor in membranes should proceed with caution. In particular, the spatial arrangement of oligomerized receptor molecules in micelles may differ from arrangement in lipid bilayers. The lipid matrix may have a profound influence on oligomerization.

    The ultimate question to answer is how oligomerization alters receptor function. This will have to be addressed in a future study.

    We could not agree more. We address the concern regarding the translatability of properties of membrane proteins in detergent micelles to the cellular context in our response to Reviewer 1. In short, we believe the general propensity for A2AR to form dimers/oligomers and the role of the C-terminus will hold in the cellular context. However, even if it does not, given that biophysical structure-function studies of GPCRs are conducted in detergent micelles and other artificial environments, it is critical to understand the role of the C-terminus in the oligomerization of reconstituted A2AR in detergent micelles. How oligomerization alters receptor function is a question that is always on our mind and should be the the focus of future studies. Indeed, it has been demonstrated that truncation of the A2AR C-terminus significantly reduces receptor association with Gαs and cAMP production in cellular assays (Koretz et al, Biophys J 2021, https://doi.org/10.1016/j.bpj.2021.02.032). The results presented in this manuscript, which have demonstrated the impact of C-terminal truncation on A2AR oligomerization, will offer critical understanding for such study of the functional consequences of A2AR oligomerization.

    Reviewer #3 (Public Review):

    The work of Nguyen et al. demonstrates the relevant role of the C-terminus of A2AR for its homo-oligomerization. A previous work (Schonenbach et al. 2016) found that a point mutation of C394 in the C-terminus (C394S) reduces homo-oligomerization. Following this direction, more mutants were generated, the C-terminus was also truncated at different levels, and, using size-exclusion chromatography (SEC), the oligomerization levels of A2AR variants were assessed. Overall, these experiments support the role of the C-terminus in the oligomerization process. MD studies were performed and the non-covalent interactions were monitored. To 'identify the types of non-covalent interaction(s)', A2AR variants were also analysed modulating the ionic strength from 0.15 to 0.95 M. The C-terminus peptides were investigated to assess their interaction in absence of the TM domain.

    The SEC results on the A2AR variants strongly support the main conclusion of the paper, but some passages and methodologies are less convincing. The different results obtained for dimerization and oligomerization are low discussed. The MD simulations are performed on models that are not accurately described - structural information currently available may compromise the quality of the model and the validity of the results (i.e., applying MD simulations to low-resolution models may not be appropriate for the goal of this analysis, moreover the formation of disulfide bonds cannot be simulated but this can affect the conformation and consequently the interactions to be monitored). Although the C-terminus is suggested as 'a driving factor for the oligomerization', the TM domain is indeed involved in the process and if and how it will be affected by modulating the solvent ionic strength should be discussed.

    We thank the reviewer for the overall positive assessment and critical input. We will respond to the comments as followed.

    The qualitative trend for dimerization is consistent with that for oligomerization, as demonstrated in Figs. 2A, 3B, and 5. For example, a reduction in both dimerization and oligomerization was observed upon C394X mutations (Figure 2A), as well as upon systematic truncations (Figure 3B), while very similar trends were seen for the change in the dimer and oligomer levels of all four constructs upon variation of ionic strength (Figure 5).

    We agree that the experimental observation and MD simulation only incompletely describe the state of the A2AR dimer/oligomer. For example, we discover the impact of ERR:AAA mutations of the C-terminus (Figure 3C) on oligomer formation, but do not know whether this segment interacts with the TM domain or C-terminus of the neighboring A2AR. MD simulations suggest that the inter-protomer interface certainly involves inter-C-termini contact. We also mention that the A2AR oligomeric interfaces could be asymmetric, suggesting that the C-terminus can interact with other parts of the receptor, including the TM domain. However, we do not have evidence that the TM domain directly interact with each other to stabilize A2AR oligomers, and thus cannot discuss the effect of the solvent ionic strength on how the TM domain contributes to A2AR oligomerization. We minimize such discussion in our manuscript because we have incomplete insights. What we can say is that multiple and weak inter-protomer interactions that contribute to the dimer and oligomer interface formation prominently involve the C-terminus. Ultimately, the structure of the A2AR dimer/oligomer needs to be solved to answer the reviewer’s question fully.

    With respect to the validity of our model, we restricted ourselves to using the best-available X-ray crystal structure for A2AR. Since this structure (PDB 5G53) does not include the entire C-terminus, we resorted to using homology modeling software (i.e., MODELLER) to predict the structures of the C-terminus. In our model, the first segment of the C-terminus consisting of residues 291 to 314 were modeled as a helical segment parallel to the cytoplasmic membrane surface while the rest of the C-terminus was modeled as intrinsically disordered. MODELLER is much more accurate in structural predictions for segments less than 20 residues. This limitation necessitated that we run an equilibrium MD simulation for 2 µs to obtain a well-equilibrated structure that possesses a more viable starting conformation. We have included this detailed description of our model in lines 641–650. To validate our models of all potential variants of A2AR, we calculated the RMSD and RMSF for each truncated variant. Our results clearly show that the transmembrane helical bundle is very stable, as expected, and that the C-terminus is more flexible (see figure below). This flexibility is somewhat consistent for lengths up to 359 residues, with a more noticeable increase in flexibility for the 394-residue variant of A2AR.

    Root mean square fluctuation (RMSF) from sample trajectories of truncated variants modeled from the crystal structure of the adenosine A2AR bound to an engineered G protein (PDB ID 5G53), and the root mean square deviation (RMSD) of the C-terminus of each variant starting from residue 291.

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  2. Evaluation Summary:

    The work will be of interest to anyone interested in signal transduction, and especially those studying GPCRs and membrane receptors. Dimerization and oligomerization of G protein-coupled membrane receptors (GPCRs) are expected to be critical for receptor function. This intriguing study helps understand the usually underestimated role of GPCR soluble domains. The authors discovered that oligomerization of a GPCR can be mediated by multiple weak interactions between soluble domains that are "tunable" by environmental factors, thus possibly modulating the function of this important class of membrane receptor proteins. What remains to be shown is how oligomerization alters receptor function.

    (This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 and Reviewer #2 agreed to share their name with the authors.)

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  3. Reviewer #1 (Public Review):

    The physical principles underlying oligomerization of GPCRs are not well understood. Here, authors focused on oligomerization of A2AR. They found that oligomerization of A2AR is mediated by the intrinsically disordered, extramembraneous C-terminal tail. Using experiment and MD simulation, they mapped the regions that are responsible for oligomerization and dissected the driving forces in oligomerization.

    This is a nice piece of work that applies fundamental physical principles to the understanding of an important biological problem. It is a significant finding that oligomerization of A2AR is mediated by multiple weak interactions that are "tunable" by environmental factors. It is also interesting that solute-induced, solvent-mediated "depletion interactions" can be a key driving force in membrane protein-protein interactions.

    Although this work is potentially a significant contribution to the fields of GPCRs and molecular biophysics of membrane proteins in general, there are several concerns that would need to be implemented to strengthen the conclusions.

    1. How reasonably would the results obtained in the micellar environment be translated into the phenomenon in the cell membranes?

    1a) Here authors measured oligomerization of A2AR in detergent micelles, not in the bilayer or cellular context. Although the cell membranes would provide another layer of complexity, the hydrophobic properties and electrostatics of the negatively charged membrane surface may cooperate or compete with the interactions mediated by the C-terminal tail, especially if the oligomerization is mediated by multiple weak interactions.

    1b) Related to the point above (1a), I wonder if MD simulation could provide an insight into the role of the lipid bilayer in the inter- or intra-molecular interactions involving the tail. Although the neutral POPC bilayer was employed in the simulation, the tail-membrane interaction may affect oligomerization since the tail is intrinsically disordered and possess a significant portion of nonpolar residues (Fig. S4).

    1. Ensuring that the oligomer distributions are thermodynamic products.

    Since authors interpret the SEC results on the basis of thermodynamic concepts (driving forces, depletion interactions, etc.), it would be important to verify that the distribution of different oligomeric states is the outcome of the thermodynamic control. There is a possibility that the distribution is the outcome of the "kinetic trapping" during detergent solubilization.

    1. The claim that the C-terminal tail is engaged in "cooperative" interactions is too qualitative (p. 11 line 274, p.12 line 279 and p.18 line 426).

    This claim seems derived from Fig. 3b and Figs. 4b-c. However, the gradual decrease in the dimer level and the number of interactions may indicate that different parts in the C-terminal tail contribute to dimerization additively rather than cooperatively. The large decrease in the number of interactions may stem from the large decrease in the length (395 to 354). Probably, a more quantitative measure would be the number of interactions (H-bonds/salt bridges) normalized to the tail length upon successive truncation. Even in that case, the polar/charged residues would not be uniformly distributed along the primary sequence, making the quantitative argument of cooperativity challenging.

    1. On the compactness and conformation of the C-terminal tail:

    Although the C-terminal tail is known as "intrinsically disordered", the results seem to indicate that its conformation is rather compact (or collapsed) with a number of intra- and intermolecular polar interactions (Fig. 4) and buried nonpolar residues (Fig. 6), which are subject to depletion interactions (Fig. 5). This raises a question if the tail indeed "intrinsically disordered" as is known. Recent folding studies on IDPs (Riback et al. Science 2017, 358, 238-; Best, Curr Opin Struct Biol 2020, 60, 27-) suggest that IDPs are partially expanded or expanded rather than collapsed.

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  4. Reviewer #2 (Public Review):

    The authors expressed A2A receptor as wild type and modified with truncations/mutations at the C-terminus. The receptor was solubilized in detergent solution, purified via a C-terminal deca-His tag and the fraction of ligand binding-competent receptor separated by an affinity column. Receptor oligomerization was studied by size exclusion chromatography on the purified receptor solubilized in a DDM/CHAPS/CHS detergent solution. It was observed that truncation greatly reduces the tendency of A2A to form dimers and oligomers. Mechanistic insights into interactions that facilitate oligomerization were obtained by molecular simulations and the study of aggregation behavior of peptide sequences representing the C-terminus of A2A. It is concluded that a multitude of interactions including disulfide linkages, hydrogen bonds electrostatic- and depletion interactions contribute to aggregation of the receptor.

    The general conclusions appear to be correct and the paper is well written. This is a study of protein association in detergent solution. It is conceivable that observations are relevant for A2A receptors in cell membranes as well. However, extrapolation of mechanisms observed on receptor in detergent micelles to receptor in membranes should proceed with caution. In particular, the spatial arrangement of oligomerized receptor molecules in micelles may differ from arrangement in lipid bilayers. The lipid matrix may have a profound influence on oligomerization.

    The ultimate question to answer is how oligomerization alters receptor function. This will have to be addressed in a future study.

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  5. Reviewer #3 (Public Review):

    The work of Nguyen et al. demonstrates the relevant role of the C-terminus of A2AR for its homo-oligomerization. A previous work (Schonenbach et al. 2016) found that a point mutation of C394 in the C-terminus (C394S) reduces homo-oligomerization. Following this direction, more mutants were generated, the C-terminus was also truncated at different levels, and, using size-exclusion chromatography (SEC), the oligomerization levels of A2AR variants were assessed. Overall, these experiments support the role of the C-terminus in the oligomerization process. MD studies were performed and the non-covalent interactions were monitored. To 'identify the types of non-covalent interaction(s)', A2AR variants were also analysed modulating the ionic strength from 0.15 to 0.95 M. The C-terminus peptides were investigated to assess their interaction in absence of the TM domain.

    The SEC results on the A2AR variants strongly support the main conclusion of the paper, but some passages and methodologies are less convincing. The different results obtained for dimerization and oligomerization are low discussed. The MD simulations are performed on models that are not accurately described - structural information currently available may compromise the quality of the model and the validity of the results (i.e., applying MD simulations to low-resolution models may not be appropriate for the goal of this analysis, moreover the formation of disulfide bonds cannot be simulated but this can affect the conformation and consequently the interactions to be monitored). Although the C-terminus is suggested as 'a driving factor for the oligomerization', the TM domain is indeed involved in the process and if and how it will be affected by modulating the solvent ionic strength should be discussed.

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