PBS and OCP Reconstitution and Protein Cross-
Linking Experiment. The PBS−OCP complexes were
prepared in 0.8 M phosphate buffer (pH 7.0) by illumination
of isolated PBS with 2000 μmol of photons of white light m−2
s−1 for 10 min at 23 °C in the presence of the OCP (OCP:PBS
ratio of 40:1).15 To separate the resulting PBS−OCP
complexes from the unbound OCP after illumination, the
sample was loaded onto a 100 kDa filter (Millipore Amicon
Centrifugal Filters, Billerica, MA) and centrifuged at 4000g and
23 °C for 20 min. Fresh 0.8 M phosphate buffer was applied
three times to dilute the retentate. The composition of the blue
retentate was checked by absorption and fluorescence emission
spectroscopies (Figure S4 of the Supporting Information). The
concentration of PBS was calculated on the basis of the molar
absorptivity formula.15 The cross-linking experiment and LC−
MS/MS were used as previously described with minor
changes.33 The PBS−OCP sample was resuspended at 0.1
μM based on the PB concentration calculation. BS3 and DSS
cross-linking was performed according to the manufacturer’s
protocol followed by desalting using Zeba columns (Thermo
Scientific, Waltham, MA) with minor modifications. Modified
samples were precipitated using acetone, and the resuspended
samples were directly subjected to trypsin digestion.