Dual-targeted labeling of proteins using cysteine and selenomethionine residues

A new strategy for dual site-selective labeling of proteins that uses metabolically incorporated selenomethionine as a target for covalent modification by iodoacetamide derivatives, forming selenonium salts, is described. In the absence of free cysteine, labeling is specific and efficient. Dual-targeted labeling of a protein can be achieved with combinations of unique cysteine and methionine residues, if the cysteine is labeled first with a maleimide or another reagent that does not react with the selenomethionine. The method should be useful in biophysical applications such as fluorescence energy transfer.