Functions as a scaffold protein for the assembly of beta-carboxysomes, initiates carboxysome assembly via its N-terminal domain binding to CcaA, CcmK and CcmL. Binds HCO(3)-, suggesting it may play a role in the activity or regulation of bicarbonate dehydration (PubMed:17993516). Also initiates carboxysome assembly by coalescing RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) via its SSU-like domains. Produced as a full-length and a shorter form; both forms are required for correct carboxysome assembly and growth (By similarity). Despite its strong similarity to gamma-class carbonic anhydrase (CA) it does not have detectable CA activity (Ref.2, PubMed:17993516). 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. CcmM73 also interacts with CcmK proteins and CcmL directly. Shell formation requires 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. Probably forms homotrimers (Probable). Full length CcmM interacts with CcaA, CcmK1, CcmK2, CcmK4, CcmL, CcmN and itself, while the N-terminus of CcmM (first 249 residues) only interacts with CcaA, CcmM and CcmN. A probable CcmM-CcaA-CcmN complex as well as a CcaA-RuBisCO-CcmM complex can also be isolated (PubMed:17993516). Interacts with full-length CcaA and the first 220 residues of CcaA; surface residues Gln-177 to Gln-188 are responsible in part for binding (PubMed:27729545). This cyanobacterium makes beta-type carboxysomes. Full length protein associates with the shell portion of carboxysomes, CcmM52 associates with both the soluble and shell portion of 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. 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. Multiple forms of the protein of 73 (full length), 62, 52 (the most predominant form) and 36 kDa are seen even in the presence of protease inhibitors. CcmM52 interacts with CcaA. Belongs to the gamma-class carbonic anhydrase family.