| In Synechocystis sp. PCC6803, the ccm-carboxysome components were found to form a multi-protein complex that included CcaA, CcmM, CcmK, CcmL, and CcmN. Yeast two-hybrid and co-immunoprecipitation analyses revealed that CcmM was the central focus of this multi-protein complex. The gamma-CA like domain of CcmM was required for protein-protein interaction with CcaA and CcmN, as well as formation of CcmM multimeric complexes. Two-hybrid and mass spectrometric analyses demonstrated that a short region of the CcaA C-terminus was necessary for interaction with an additional CcaA monomer and with CcmM, as well as for maintenance of CA catalytic activity. The association of CcmM and CcaA with the pelletable carboxysome shell fraction indicated that the CcmM multi-protein complex was localized exclusively to the carboxysome periphery. Collectively, these findings indicated a new and emerging carboxysome model where the shell-localized CcaA catalyzes the formation of CO2 then directing this substrate to RuBisCO for fixation. |
