(A) MS/MS of peak at 2063 Da, containing a label derived from F2CTP. The y series is marked, and corresponds to the C-terminal peptide of RTPR (DLELVDQTDCEGGACPIK). The absolute MWs detected are shifted +43 Da relative to the acetamide labeled C-terminal peptide (Figure S1), suggesting alkylation at cysteine by a group 43Da larger than acetamide. The modification is present to some extent on each cysteine, as indicated by the appearance of the y series of the normal peptide after C731 is cleaved (indicated at 831, 702, 645, 588 and 517 Da).