Functions as a scaffold protein for the assembly of beta-carboxysomes, initiates carboxysome assembly by coalescing RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS). Produced as a full-length and a shorter form; both forms are required for correct carboxysome assembly and growth. Beta-carboxysome assembly initiates when soluble RuBisCO is condensed into a liquid matrix in a pre-carboxysome by the RbcS-like domains of probably both forms of CcmM. CcmN interacts with the N-terminus of full length CcmM, and then recruits the shell proteins (CcmK) via CcmN's encapsulation peptide. Shell formation requires both CcmK proteins and CcmO. CcmL caps the otherwise elongated carboxysome. Once fully encapsulated carboxysomes are formed, they migrate within the cell probably via interactions with the cytoskeleton. Probable homotrimer (By similarity). Full length protein (M70) interacts with CcmN. The C-terminal RbcS-like domains (SSUL) bind to holo-RuBisCO, as does the M46 short form (By similarity). This cyanobacterium makes beta-type carboxysomes. The N-terminus has a gamma-class carbonic anhydrase-like domain (CA), while the C-terminus has 4 repeats that are similar to the small subunit of RuBisCO (rbcS), called SSUL (Probable). The SSUL are connected by flexible linkers. The N-terminal domain forms a scaffold on which CA and other carboxysomal proteins assemble, while the C-terminus binds RuBisCO (By similarity). Identified as 2 proteins of 70 and 46 kDa in gels. They are probably the result of alternative translation initiation. Single gene deletion does not form carboxysomes, and does not grow in normal air but in 2% CO(2), called a high-CO(2) requiring phenotype, HCR. Belongs to the gamma-class carbonic anhydrase family.