Given that the major goal of these experiments is to map the
interaction between the OCP and PBS, we focused on the
cross-link peptides between the OCP and PBS (Table S3 of the
Supporting Information). Among the cross-link peptides, the
lysine (K) cross-links are OCP K167−ApcB K58, OCP K249−
ApcB K26, and OCP K170−ApcE K4 (Table S3 of the
Supporting Information). All these lysines are solvent-exposed,
according to the crystal structures of APC [Protein Data Bank
(PDB) entry 4F0U] and the OCP (PDB entry 3MG1) from
Synechococcus elongatus sp. PCC 7942 and Synechocystis sp. PCC
6803, respectively; the former protein sequence shares a high
degree of identity with that of Synechocystis sp. PCC 6803.