Assembly mechanism of the β-carboxysome shell mediated by the chaperone CcmS

Carboxysomes are self-assembled bacterial microcompartments (BMCs) that encapsulate the enzymes RuBisCO and carbonic anhydrase into a proteinaceous shell, enhancing the efficiency of photosynthetic carbon fixation. The chaperone CcmS was reported to participate in the assembly of β-carboxysomes; however, the underlying molecular mechanism remains elusive. We report the crystal structure of CcmS from Synechocystis sp. PCC 6803, revealing a monomer of α/β fold. Moreover, its complex structures with two types of BMC hexamers, CcmK1 homohexamer and CcmK1-CcmK2 heterohexamer, reveal a same pattern of CcmS binding to the featured C-terminal segment of CcmK1. Upon binding to CcmS, this C-terminal segment of CcmK1 is folded into an amphipathic α-helix protruding outward that might function as a hinge to crosslink adjacent BMC-H hexamers, thereby facilitating concerted and precise assembly of the β-carboxysome shell. Deletion of the ccmS gene or the 8-residue C-terminal coding region of ccmK1 resulted in the formation of aberrant and fewer carboxysomes, suppressed photosynthetic capacity in Synechocystis sp. PCC 6803. These findings enable us to propose a putative model for the chaperone-assisted assembly of β-carboxysome shell and provide clues for the design and engineering of efficient carbon fixation machinery.