The GPCR-G complex: the canonical structure of Cell Signaling (with interactive guide)
So noobs, why is it important? According to some estimates, some 30% of our proteins are membrane proteins and the reason that they are so many is that membrane proteins are where most of the signal processors between our cells are found.
Our multi-cellularity depends on a constant cascade of information, as signaling molecules flow back and forth across different types of cell. Spatially, it's much easier for a signaling molecule to hit something on the 2D surface of the membrane than to tunnel through and search for the target within the body of a cell. Signals that hit the membrane, propagate through the membrane courtesy of an adaptor molecule. The signaling molecule, an agonist, will hit the adaptor from the extra-cellular side, which activates the adaptor. The adaptor will in turn activate other proteins on the inside of the cell, which in turn will cause the cell to change. The precision of the adaptor in converting outside to inside signals communicates information between cells. These signal adaptors thus encode our multi-cellularity as organisms.
The most common architecture for such signaling adaptors is the G Protein-Coupled Receptor (GPCR). They include receptors that are responsible for seeing, smelling, hormones and neurotransmitters. The GPCR binds molecules from the outside of the cell, and then switches on a matching G-protein inside of the cell. The G-proteins are so called because they use GTP as a timer (the same G in DNA). The activated GPCR forces the G-protein to bind a new GTP molecule, which activates the G-protein. When the G-protein is released, it will then bind to all sorts of targets, setting off new cell states. The G-protein shuts down when the GTP spontaneously breaks down into the rather inert form of GDP. It will then wait for the next GPCR activation.
This GPCR-G interaction is thus the cornerstone interaction of cell signaling.
Now of course, we'd like to fuck around with this process, and we do, considering that over 30% of modern pharmaceutical drugs target a GPCR protein in our bodies. But designing such drugs is an imprecise process, full of random trial-and-error procedures. If we were know to how the GPCR-G interaction happens at atomic detail, then we may understand how these drugs actually work, with the hope of designing better ones.
The Nature paper from the Kobilka lab at Standford reports the crystallization of the GPCR complexed to a G protein [3sn6]. It's almost academic which GPCR protein it is, but it's the Β2-adrenergic receptor complex, which binds to to adrenalin.
[For those of you with Chrome/Safarit/Firefox(recent) you will see an interactive widget for the structure below. Click the arrows on the top, or go straight to the Jolecule page where you can add your own annotations]
What the Kobilka lab had to do to get the thing to crystallize is nothing short of phenomenal. Membrane protein crystallography is hard enough as it is. The greasy surface of the membrane proteins designed to slide into the greasy membrane do not crystallize easily. Here, they had to crystallize a membrane protein stuck to many different cytosolic proteins, which makes it even harder still. They had to identify an agonist (or trigger for binding) that would work in the mutant complex. They replaced the entire unstructured C-terminal of the GPCR with a whole lysozyme insert. They must have sieved through an interminable number of trials to find a nanobody (a truncated antibody) that would bind the different pieces of the G-protein sufficiently so that the complex would survive the crystallization process. As well, they would have to induce the right kind of lipid to form an intact micelles around the membrane region.
The structure is beautiful, in the sense that it fits what the biochemistry tells us and gives a clear mechanical view of how it works. One fantastic result is that we see that the G-protein is splayed open when bound to the GPCR. The two domains of Gα – Gα-helical cause it's mainly helical and Gα-Ras cause it's a Ras-like domain – are found completely opened. The GTP binding site is completely exposed.
In the unbound form of Gα [1AZT], the binding site is complete closed over as the two domains, Gα-helical and Gα-Ras are closed over each other.
This structure also shows how the G-protein is inserted into the GPCR, especially the orientation of the A5-helix. This rationalizes known mutation experiments, and suggests many new ones. This structure provides incredible detail, but more importantly, it provides a molecular template to study all the other myriad signals that flow in-and-out of our cells. It is sobering indeed to gaze at the interaction that allowed our unicellular ancestors to cross-talk their way into our multi-cellular glory.