N-Hydroxysuccinimide (NHS)-esters are widely used to label proteins nonselectively on free amino groups. Such broad labeling can be disadvantageous because it can interfere with protein structure or function and because stoichiometry is poorly controlled. Here we describe a simple method to transform NHS-esters into site-specific protein labeling on N-terminal Cys residues. MESNA addition converts NHS-esters to chemoselective thioesters for N-Cys.modificatiOn. This labeling strategy was applied to clarify mechanistic features of the ubiquitin E3 ligase WWP2 including its interaction with one of its substrates, the tumor suppressor PTEN, as well as its auto-ubiquitination molecularity. We propose that this convenient protein labeling strategy will allow for an expanded application of NHS-esters in biochemical investigation.