Published 1 July 2013 by Ashutosh Jogalekar
Brian and Bob’s GPCR symphony
Brian Kobilka and Robert Lefkowitz received the 2012 Nobel Prize in chemistry for telling us how the molecular musicians in living organisms play the tune of life. Almost any molecular mechanism in our body provides an illuminating example of a superbly choreographed ballet, but Kobilka and Lefkowitz shed light on the motions of one of the most important class of signaling artists – G Protein-Coupled Receptors or GPCRs.
Three key players make up the intricate dance which these all-important proteins orchestrate. The first component is a tiny molecular messenger, a small molecule like adrenaline or even a single photon which whispers in the innards of seven winding alpha helices which span the membranes of every conceivable kind of cell in the human body. Like listening stations in a foreign land, these helices have part of their ear turned outward and part inward. The part that’s turned outward patiently lets the messenger tickle its precisely grouped set of amino acids. The part that’s turned inward binds to proteins called G-proteins and arrestins. What follows from the simple interaction of a tiny molecule on the cell’s exterior is what in part bagged Kobilka and Lefkowitz the prize and what has opened up a whole new world of understanding in GPCR research and biological signaling mechanisms in general. Through decades of patient, often thankless and sometimes heroic efforts, Kobilka and Lefkowitz told us the story of how cells communicate knowledge of the external world into their interior, a paradigmatic development that was surely a foundational event in the evolution of life.
The interaction between the small molecule and the outer ear of GPCRs sets into motion a series of subtle but key movements whose culmination is an elemental physiological response like the turning on or off of key genes involved in a variety of critical processes, from sensing environmental stimuli to reacting to them. Nailing down the molecular details of this response and creating a movie-like set of snapshots of the events involved in them was a technical tour de force that made the prize for Kobilka and Lefkowitz a shoo-in. Lefkowitz pioneered the modern study of GPCRs by isolating the gene for the adrenaline receptor and deciphering the universal seven transmembrane alpha helical structure of the proteins. Drilling down into the details was a task that Kobilka – who started as a GPCR enthusiast in Lefkowitz’s lab as a postdoc – pursued with fervent dedication for twenty years. The culmination of his group’s effort was a series of breathtaking atomic-level crystalline views of the adrenaline receptor caught in various acts, all published in a set of pioneering papers in Science and Nature during the last decade.
The basic problem with crystallizing GPCRs was that they rapidly disintegrated when wrenched away from the protective embrace of the lipid bilayer that makes up all cells. The solution was to find out a set of detergents – after making your way through an almost countless number of trials – that would serve as a proxy for the lipid’s nurturing environment. The study of GPCRs is made even more complicated by the fact that they exist in several states. In addition these multiple states can be activated or deactivated by several kinds of small molecules called agonists (activators) antagonists and inverse agonists (deactivators). The key challenge that Kobilka’s group overcame was to find out the right conditions for crystallizing GPCRs and studying them at leisure in all their forms. Coaxing a lone GPCR to crystallize is daunting enough, but getting it to crystallize while being bound to an agonist seemed insurmountable; it would be much like trying to freeze a stick of dynamite for study after the fuse has been lit. But Kobilka’s lab found several novel strategies to accomplish this. Two of the most novel involved using an antibody from a llama to tightly bind and stabilize the protein, and using a special kind of lipidic material to make sure that the protein does not fall apart. Starting from a crystal structure of a lone GPCR and then one bound to a deactivating antagonist, Kobilka and his team finally solved the structure of the adrenaline receptor bound to an agonist. This was something that scientists had tried with all their determination, to no success.
But it was what Kobilka and Lefkowitz’s groups did after this that sealed their fate as Nobel Laureates. The real meat in the workings of GPCRs is concealed in the intimate intracellular side of the proteins which faces the critical group of other signaling proteins and small molecules that can turn genes on and off. The fate-determining event that conveys the message whispered by the small molecule messenger is the interaction of the inner ear of the protein with the molecular entity featured in its very name – a G-protein. G-proteins are complex assemblies of subunits that are ubiquitously involved in signaling events; in fact their discovery had even won Alfred Gilman and Martin Rodbell a Nobel Prize, one which in fact made Lefkowitz feel a little left out. But even after their discovery, nobody knew how G-proteins interacted with their receptors to actually bring about gene expression and physiological responses. In 2011 Kobilka capped his stellar efforts at GPCR crystallography with the first structure of a GPCR bound to a G-protein. The structure shed valuable light on the precise details of how GPCRs and G-Proteins interact with each other, followed by the dissociation of the G-protein and the activation of a variety of other macromolecules whose net action results in gene expression. This development followed on the heels of another key discovery by Lefkowitz’s group – that of arrestins, proteins that are essentially responsible for taming and recycling overactive GPCRs that are unduly sensitized by interaction with G-Proteins.
In over three decades of patient work, Kobilka and Lefkowitz thus unravelled the whole GPCR symphony, ranging from the initial binding of a messenger to the GPCR, through the transmission of this signal to the interior and the movement of the alpha helices, finally culminating in the activation of a G-protein and the desensitization of the GPCR by arrestins. A better example of going all the way to the finish and tying up the loose ends would be hard to find in the annals of the Nobel Prize.
Does this mean that we now know everything there is to know about GPCRs? Far from it. One of the constant things about science is that it always opens up more new questions than it answers. For one thing, there are more than 800 GPCRs involved in every kind of important physiological response. In addition there are at least three major classes. All the work by Kobilka and Lefkowitz has been done on Class A GPCRs; Class B GPCR structures have only just started yielding their secrets and Class C is barely on the horizon.
More importantly, the last two decades’ worth of work by Kobilka, Lefkowitz and others on GPCRs have resulted in the discovery of an absolutely fascinating and potentially very deep-seated phenomenon called functional selectivity or biased agonism. Biased agonism happens when two small molecules that are both agonists nonetheless activate their recipient GPCR differently and lead to very different physiological responses. Biased agonism results from very subtle differences between the interactions of two agonists (or antagonists for that matter) with the proteins that are still somehow amplified enough to cause dissimilar signaling. If GPCRs have been nature’s principal tools for molecular homeostasis, functional selectivity is a fine scalpel that attests to the power of evolution in being detail-oriented. What is interesting is that functional selectivity is turning out to be a potential driver of signaling in other kinds of proteins, including ion channels and nuclear receptors (proteins which interact with hormones). The best science not only illuminates its own contours but also other corners of the great void, and Kobilka and Lefkowitz’s GPCR studies may turn out to be prophetic starting points for understanding every single other signaling mechanism in living organisms. If this indeed turns out to be the case, then their work would end up with honors even greater than the Nobel Prize.