HEK293T cells that transfected with plasmid encoding human FLAG-Granzyme A (Sino Biological, HG13325-NF) using lipofectamine 3000 for 24 h were treated with 4-OI (0.5 mM) for 6h.Following treatment, cells were lysed in lysis buffer and immunoprecipitated with anti-FLAG antibody and protein A/G agarose beads. Next the beads were washed three times with 1ml lysis buffer after centrifuging at 400g for 2 min. The immune complexes were eluted by addition of 40 ml lysis buffer, dissolved in 1x SDS sample buffer and boiled for 5 min. The samples were separate by SDS-PAGE and subsequently probed with Coomassie blue. The corresponding bands for FLAG-Granzyme A were excised from the gel and subjected to in-gel digest with trypsin. Briefly, the gel slices were cut into smaller pieces before reduction with dithiothreitol (10 mM) and then dehydrated in 100% acetonitrile. Gel slices were digested with trypsin overnight at 37℃. Peptides were eluted from the gel pieces following protease digest and dried down completely in a vacuum centrifuge. Samples were analyzed in a Q Exactive TM Plus (Thermo Fisher) coupled online to UPLC after the nanospray ionization source. MS data was analyzed with Proteome Discoverer 1.3 and tandem mass spectra were searched against the UniProt database. Trypsin/P is designated as a cleavage enzyme that allows up to two deletion cleavage. Precursor mass tolerance was set to 10 ppm, while fragments were detected with a tolerance of 0.02 Da.