Mass Spectrometry (MS) of the unmodified and TPCK-modified Nef-His proteins. (A) MS- determination of the molecular weight of the unmodified (TPCK-) and modified (TPCK+) Nef-His proteins. (B) Tryptic mapping of Nef-His proteins by MS. Unmodified (top panel) or TPCK-modified Nef-His proteins (bottom panel) were excised from SDS PAGE gels, digested by trypsin and injected into Micromass (Waters) for MS (MALDI-ToF). Arrows indicate the tryptic peptides containing cysteine: P1263 (C142), P1430 (C206) and P4787 (C55) from unmodified Nef-His (top panel), and P1263 (C142), P1745 (C206) and P5100 (C55) from the TPCK-modified Nef-His (bottom panel). (C) MS sequencing of the modified C-terminal peptide (P1745). Residue 10B is the modified Cys206. Three residues Glu, Leu and Glu between Nef and His-tag are translated from the vector poly-linker region. Note, different sensitivity scales are used to show the unmodified C55 (P4787) and TPCK modified C55 (P5100).