Evidence from native MS that the transient conversion of the
OCPo to the OCPr involves the monomerization of the OCP,
which is competent to bind to the PBS for fluorescence
quenching and photoprotection, is presented here. Protein
cross-linking analysis further indicates that the N-terminal
domain of the OCP is buried between the APC660 trimer and
the APC680 trimer in the PBS core. This structural information
about the OCP provides a basis for future studies of the
detailed interactions between the 3′-hECN and chromophores
in the PBS core and the regulation of light harvesting.
The potential of MS-based approaches in studies of OCPrelated
photoactivation has been demonstrated. Although MSbased
approaches cannot provide high-resolution structural
information, the speed and sensitivity of MS will further benefit
the investigation of OCP-related photoactivation in cyanobacteria