At long last, a 2nd GPCR crystal structure

G-protein coupled receptors, or GPCRs, are a key class of eukaryotic membrane receptors. Roughly 50% of all small molecule therapeutics target GPCRs. Vision, smell & some of taste uses GPCRs. Ligands for GPCRs cover a wide swath of organic chemical space, including proteins, peptides, sugars, lipids and more.

Crystal structures are spectacular central organizing models for just about everything you can determine about a protein. Mutants, homologs, interactors, ligands -- if you have a structure to hang them on, understanding them becomes much easier. For drug development a 3D structure can be powerful advice for chemistry efforts, suggesting directions to build out a molecule or to avoid changing.

Because they are large, membrane-bound proteins with lots of floppy loops, GPCRs are particularly challenging structure targets. Efforts to build homology models relied on bacteriorhodopsin, which is not a GPCR but has the seven transmembrane topology of GPCRs. The first GPCR structure was finally published in 2000, of bovine rhodopsin. Cow rhodopsin has a significant advantage in that large quantities can be purified from an inexpensive natural source, cow eyes.

Since then, published crystallography of GPCRs has been restricted to further studies on rhodopsin (e.g. this mutant study). Rumors of further structures at private groups would periodically surface, but given the lack of publications & the high PR value of a publication, it seems likely these were just rumors. Now, after 7 years, the drought has been ended with a flurry of papers around the structure of a beta adrenergic receptor, the target of beta blockers.

The papers share a number of co-authors but describe two different approaches to solving the GPCR crystallization problem. For the beta-2-adrenergic receptor, a key problem is a floppy intracellular loop. In the pair (here & here) of papers online at Science, the troublesome 3rd intracellular loop is largely replaced with T4 lysozyme, a protein which has been crystallized ad infinitum. In the Nature paper & a Nature Methods paper describing the method, the intracellular loop is stabilized with an antibody raised against it.

The abstracts hint that B2AR and rhodopsin are strikingly different in some important ways, underlining the need for multiple crystal structures for a family -- with only one, it is impossible to determine what is general and what is idiosyncratic. Indeed, one of the papers reports that published homology models of B2AR were more similar to rhodopsin than the new B2AR structure.

Will these new approaches herald a flurry of GPCR structures? Perhaps, but they hint at what a hard slog it may be. A host of additional challenges were faced, such as the crystals being so transparent it was hard to position them in the beam. Will each GPCR present its own challenges? Only time will tell.