Bodipy 576/589 labeling of VCB VCB was labeled with Bodipy 576/589 following protocol reported previously . Briefly, the VCB complex was mixed with Bodipy 576/589 NHS ester in a 20:1 molar ration and incubated at room temperature (protect from light) for 2h in reaction buffer (0.1 M sodium phosphate, 75 mM KOAc, 2 mM DTT, pH 7.4). The reaction was quenched by diluting 10 times with reaction buffer and the unreacted dye was removed with a PD-10 MiniTrap desalting column (GE Healthcare) equilibrated with 100 mM Bis-Tris pH 7.0, 100 mM NaCl, 1 mM DTT, pH 7. The eluted labeled protein solution was collected and concentrated with Pierce Concentrator, 3K MWCO (Thermo scientific).