It's hard to think of a more important class of drug targets than the G-protein coupled receptors (GPCRS). And back about fifteen years ago, I thought I had a reasonable understanding of how they worked. I was quite wrong, even given the standards of knowledge at the time, but since then the GPCR world has become gradually crazier and crazier.
The classic way of thinking about these receptors is that they live up on the cell surface, with part of the protein on the outside and part on the inside. The inside face is associated with various G-proteins, and the outside face has a binding site for some sort of signaling molecule. If the right molecule shows up and slots in the correct way into this binding cavity, the transmembrane helices of the protein rearrange, sliding around to change the shape and binding properties down there at the G-protein interface. This sets off some intracellular messaging - often by affecting levels of the messenger molecule cyclic-AMP. Thus is a signal from outside the cell relayed through the membrane to the inside.
Pretty nearly makes sense, doesn't it? Well, take a look at this new report from PLoS Biology. The authors rigged up living cells with a built-in fluorescent sensor system to monitor cAMP, and then studied the behavior of the thyroid-stimulating-hormone (TSH) receptor. That's a perfectly reasonable protein-ligand GPCR, but it turns out that it does things that are not (to us) perfectly reasonable.
This paper shows that when a TSH molecule binds, that the receptor gets taken back down through the membrane into the cell. That's certainly a known process (internalization), and was thought to be a regulatory process, a standard method for taking a specific GPCR out of the signaling business. Some receptors seem to do this right after they're used, and of those, some of them later resurface and some are broken up. (Other types hang around for many cycles until they're somehow worn out). But the ones that internalize quickly still set off their intracellular message before they get pulled back down. That's their purpose in life.
TSH does that. But the weird part is that the authors saw the receptor internalize along with its G-protein partners, and then continue signaling from inside the cell. Not only that, this extra signaling behavior set off somewhat different responses as compared to the first "normal" burst, and seems to be a necessary part of the usual TSH signaling pathway. It's a very odd thought, if you're used to thinking about GPCRs - it's like finding out that your cell phone works when it's turned off.
Now this sort of behavior has been demonstrated for a different class of signaling proteins (the tyrosine kinase receptors). And even GPCRs have been found, over the last few years, to be capable of setting off a different signaling regime (the MAP kinase pathway) after they've been internalized. (That's one of the weird findings of recent years that I mentioned in the introductory paragraph, and we still don't know what to do with that one as far as drug discovery goes). But everyone agreed that at least the good ol' cyclic AMP pathway worked the way we thought it did, through signaling at the cell surface, and thank goodness there was something you could still count on in this world.
Hah. Now we're going to have to see how many other GPCRs show this kind of behavior, and under what circumstances, and why. It may well turn out to be different for different cells or for different signaling ligands, or only occur under certain conditions. And we'll have to see how this relates to the other strange things that are being unraveled about GPCR behavior - they way that they can dimerize, with themselves or even other receptors, out on the cell surface, and the way that some of them seem to work in an opposite-sign signaling regime (always on, until something turns them off). Do these things still signal from beneath the waves, too?
Oh, this will keep the receptor folks busy, as if they weren't already. And, as usual when something like this shows up, it should serve as a reminder to anyone who thinks that we understand even the well-worked-out parts of cell biology. Hah!