Assembly of the B2AR-Gs complex

After obtaining the B2AR-Gs complex crystal structure, the article describes how they attempted to understand B2AR-Gs complex formation. In every experiment carried out, data obtained from the main method used is always checked and carried out in conjunction with other techniques. This is extremely important as data obtained from only once method often does not lead to conclusive evidence. In their search to determine how the B2AR-Gs complex forms they not only used various methods, they also attempted to compare their findings with analogous known structures, in this case; Rhodopsin, in a hope to find more clues into the intrecate way that the complex forms.


The initial interaction of agonist-bound B2AR and GasRas may involve the fusion of the 20 amino acids of the carboxy terminus of Gs with the carboxyl terminus of the B2AR

Examination: First attempt of producing a complex between the B2AR and a synthetic peptide representing the carboxy-terminal 20 amino acid of Gs showed no effect on receptor function. However, receptor gained a 27-fold increase in agonist binding affinity when its carboxyl terminus is fused with the peptide of 20 amino acids of the carboxyl terminus of Galphas (shown on left, B2AR is shown green here), which is only 3.5-fold smaller than the effect of agonist binding affinity observed in the B2AR-Gs complex.

 Why did they test the B2AR and GasRas interaction using a C-terminal peptide?

Transducin & rhodopsin: Transducin (Gt) is coupled with rhodopsin in light sensory signalling events. The analysis of crystal structure of metarhdopsin II alone or in complex with an 11-amino-acid C-terminal fragment derived from Gα (GαCT2) shows interactive surfaces. GαCT2 binds in a large crevice at the cytoplasmic side, and the crystal structure differences between activated and inactivated meta II have been used as a model for GPCR structure studies.

Results: Since upon agonist binding, GPCR interacts with GDP-binding Gs heterotrimer, and the association of the receptor and Gs would result in a 100-fold increase in agonist binding affinity than that of the receptor alone, the examination above showed the possible initial contacting event explanatory for this increase of binding affinity of agonist when GPCR is coupled with Gs, more importantly, for the formation of the B2AR-Gs intermediate complex formation.

The C-terminus of the alpha5-helix moves away from the beta6-strand to permit interaction with the B2AR

Evidence: The C-terminus of the Gs consist of the α4/β6 loop, the β6 β-sheet segment and the α5 helix region, which has been studied previously to show importance in stabilisation of the secondary structure of the C-terminal (The original publication here). Supported by the crystal structure of Gs and B2AR, as mentioned in our Crystal Strucutre of Gs page, the movement of the C-terminus of the alpha5-helix away from the beta6-strand may suggest how the fusion of the C-terminus of Galphas together with the C-terminus of B2AR is achieved.  

Further evidence: The carboxyl terminus of the alpha5 helix is more dynamic, therefore more available for interaction in the Gs-GDP heterotrimer than would be expected from the crystal structure of Galphas, as it is surrounded by different domains of helices and loops – looking ‘stuck’. Full analysis is produced here.

DXMS: The technique used is called deuterium exchange mass spectrometer (DXMS), which helps to analyse the structure of proteins by detecting the accessibility of the peptide amide hydrogen of a particular location when exchanging thermodynamically with deuterium present in solvent water. In a completely unstructured polypeptide sequence, peptide amide hydrogen are always maximally accessible to water and exchange at their maximal rate, whereas most amide hydrogen in structured peptides or proteins exchange much more slowly, reflecting the fact that exchange occurs only when transient unfolding fluctuations fully expose the amides to solvent water. [More details here]

 

The mechanism of releasing GDP from Galphas remains unknown

After the initiative events of interacting C-terminus of Galphas and C-terminus of B2AR, Galphas rotates to form a more extensive interaction between the B2AR ICL2 and the amino terminus of Galphas. This further facilitates conformational changes in both B2AR and GalphasRas, although when the GDP is released is unknown during this activation process of Gs.