Cys16 and Cys85 are membrane buried. Cys43, Cys51, Cys132, and Cys135 can form disulfide bonds9. Native membrane: NEM labeling was carried out with intact cell membrane, as described in Special Note 1. Triton X-100: membrane solubilization and NEM labeling were carried out at the same time, as described in the Experimental Protocol. DTT: the cysteines in hVKOR were reduced in cells before the NEM labeling.