Investigation of the protein alkylation sites of the STAT3:STAT3 inhibitor Stattic by mass spectrometry

STAT3 (Signal Transducer and Activator of Transcription factor 3) is constitutively active in a wide range of human tumours. Stattic is one of the first non-peptidic small molecules reported to inhibit formation of the STAT3:STAT3 protein dimer complex. A mass spectrometry method has been developed to investigate the binding of Stattic to the un-phosphorylated STAT3βtc (U-STAT3) protein. Alkylation of four cysteine residues has been observed with possible reaction at a fifth which could account for the mechanism of action.