Selective labelling of the hydrophobic segments of intrinsic membrane proteins with a lipophilic photogenerated carbene

Abstract

HYDROPHOBIC photogenerated reagents are attractive candidates for the labelling of those parts of membrane proteins that are buried within the lipid bilayer. Recently, nitrenes generated from aryl azides have been used to distinguish intrinsic from extrinsic membrane proteins^1–4 and to label glycophorin A in or close to the transmembrane hydrophobic sequence⁵. We have questioned the use of aryl nitrenes on the basis of their evident lack of reactivity towards carbon–hydrogen bonds in single-bilayer phospholipid vesicles⁶. Because the lipid binding sites of intrinsic membrane proteins are believed to contain chemically unreactive hydrophobic amino acids, a reactive species that is less selective than a nitrene is needed to increase the probability that all the protein segments that penetrate the lipid bilayer will be labelled. In general, carbenes are more reactive than nitrenes (ref. 7 and refs therein); indeed, we have shown that carbenes generated within lipid bilayers do insert into saturated fatty acyl chains⁸. It is important to characterise the reactivity of lipophilic photogenerated reagents with well studied membrane proteins, whose membrane-associated regions have been identified by other experiments. We report here that the carbene, ³H-adamantylidene (II), generated photochemically within biological membranes from ³H-adamantane diazirine (I), labels three such intrinsic membrane proteins in or close to the hydrophobic peptides that are believed to lie within the hydrocarbon region of the lipid bilayer.