Patent ID: 8084226

Claim:
A method for site-specific protein labeling, comprising contacting: a first protein molecule having no more than two Cys residues—a first and a second Cys residues at respective first and second sites within the first protein molecule, wherein the first Cys residue is accessible to interact with a fluorophore, and the second Cys residue is accessible to interact with a fluorophore or with a second protein molecule; a second protein, which is a blocking molecule, that is suitable to interact with the first protein molecule to selectively block the second Cys residue from fluorophore binding while leaving the first Cys residue unblocked; and a fluorophore that is suitable to bind to the first Cys residue, under conditions such that the first protein molecule and the second protein molecule bind with sufficiently high affinity to afford site-specific protection of the second Cys residue at the binding interface of the first protein molecule and the second protein molecule at a micromolar protein concentration, thereby labeling the first protein molecule specifically at the first Cys residue.