I’ve been meaning to mention this new paper, from a mixed Berkeley/UCSF team, which could open up a new area for chemical biology. If you want to label a protein, you have to have a handle to do it. “Label”, in this sense, can mean almost anything. You could be sticking a fluorescent group of some sort on there, something with an odd isotope pattern for mass spec analysis, a spin label for NMR experiments, even a tritiated or other radioactive side chain, what have you. There are no end of experiments and assays to be run once you’ve got some sort of detection technique in place, but you’ve got to get it in there first.
If you want these things covalently attached (and you often do) there are limited options, which often start and end with cysteine residues. Those SH groups are reactive enough to pick up various electrophiles, and the techniques that depend on them are legion. But there are problems. Not every protein has a Cys in a place that’s useful for your experiment, and not all Cys groups are reactive in quite the way you might need. Some proteins are ungrateful enough not to have one out on the surface at all, and other have them, but they can’t be modified without messing up some key part of their structure or function. There are other side chains that can also be modified with electrophiles, of course, but after Cys these reactivities start to get patchy and hard to predict.
This new paper details a way to modify methionine side chains, by first oxidizing them selectively. That’s a welcome addition to the toolbox, because Met is much less likely to be involved in something crucial as compared to Cys. (On the down side, Met is a much less common amino acid, but you could spin that as giving you fewer possibilities for multiple products). The method uses an oxaziridine reagent to produce a sulfimine (via the sulfoxide), and that gives you access to a whole variety of things hanging off the new nitrogen atom. For instance, you can label the Met side chain with a “click”-ready alkyne or azide group (as is demonstrated in the paper).
If there’s not a handy methionine, you can always engineer one, and the authors demonstrate this on an antibody fragment to green fluorescent protein. They were able to functionalize the new Met residue (and thus the antibody) with a variety of species and retain antibody potency, which gives you a potential new route into antibody-drug conjugates. Indeed, they went on to show just that on trastuzumab (Herceptin), engineering a methionine into it and attaching the chemotherapy agent monomethyl auristatin E. The resulting species showed enhanced activity in a cell assay, as it should.
The other thing you can do with this technique, as with the Cys labeling methods, is distinguish various types of them in the proteome. Not all cysteines are created equal, and not all methionines are, either, since they can be involved in oxidative reactions in vivo, which can be important for protein function and regulation. The oxaziridine treatment can identify the hyper-reactive Met residues, in the same way that electrophile screens can pick out the hyper-nucleophilic Cys ones.
So this looks like just the sort of thing that chemical biology people have been looking for – a new handle, with new chemistry, on a newly useful amino acid. We’ll see what people make of this now that it’s out there!