Functions as a scaffold protein for the assembly of beta-carboxysomes, initiates carboxysome assembly by coalescing RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) (Probable) (PubMed:24267892). Produced as a full-length (M58) and a shorter form (M35); both forms are required for correct carboxysome assembly and growth (PubMed:20304968). The short form is more abundant (PubMed:17675289, PubMed:20304968, PubMed:28963440, PubMed:31048338). Despite its strong similarity to gamma-class carbonic anhydrase (CA) it does not have detectable CA activity (Ref.3). The M35 isoform is able to condense RuBisCO into a liquid matrix; the presence of disulfide bonds in M35 reduces affinity for RuBisCO, while mutating all 4 Cys to Ser causes a 4-fold increase in doubling time, more than 15% increase in CO(2) requirement, and abnormal carboxysomes. 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 CcmM58 and CcmM35. CcmN interacts with the N-terminus of CcmM58, and then recruits the CcmK2 major shell protein plus other less abundant CcmK proteins via CcmN's encapsulation peptide. 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 a homotrimer (Probable). Purifies from carboxysomes in complex with both RuBisCO subunits and carbonic anhydrase (ccaA); the complex is probably associated with the carboxysome shell (PubMed:17675289). Interacts with CcmN (PubMed:22461622). Binds holo-RuBisCO (RbcL(8)-RbcS(8)) via its SSUL domains; the SSUL domain binds close to the equitorial domain of RuBisCO between RbcL dimers, with 1 M35 protein per dimer (PubMed:30675061). The short form purifies from carboxysomes in complex with both RuBisCO subunits; the complex is probably associated with the carboxysome shell. This cyanobacterium makes beta-type carboxysomes (Ref.4). Both isoforms are found equally distributed in the interior of the carboxysome (PubMed:28963440). Carboxysome size and components vary with growth conditions. When grown in ambient air at medium light (50 uE meter(-2) second(-1)) there are 719 units (both forms are included in this measure) of this protein per carboxysome, the numbers decrease under low light and high CO(2), and increase under high light (at protein level). In the N-terminus has a gamma-class carbonic anhydrase-like domain (CA), while the C-terminus has 3 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 maybe other carboxysomal proteins assemble, while the C-terminus binds RuBisCO (PubMed:8491708) (Probable). At least 2 SSUL domains are required for initiation of carboxysome assembly (PubMed:24267892). Isolated reduced and oxidized CcmM35 binds holo-RuBisCO (RbcL(8)-RbcS(8)) but neither subunit octamer alone; RuBisCO has a higher affinity for reduced M35 (PubMed:30675061). Identified as 2 proteins of 58 and 38 kDa by mass spectrometry, called M58 and M35, the shorter protein is translated starting at Val-216. Protease inhibitors do not alter the appearance of M35 (Ref.4, PubMed:20304968). In isolated carboxysomes M35 is 4-5 fold more abundant. The first amino acid (equivalent to Val-216) is not seen in Edman degradation, while Tyr-219 and Gln-222 may be post-translationally modified (PubMed:17675289). Single gene deletion does not form normal carboxysomes, and does not grow in normal air but in 2% CO(2), called a high-CO(2) requiring phenotype, HCR. CcmK2 and RuBisCO are targeted to an abnormal large polar body in some studies, RuBisCO is soluble in others (PubMed:8491708, PubMed:17675289, PubMed:20304968, PubMed:22928045, PubMed:28963440). When ccmL-ccmM-ccmN-ccmO are deleted no carboxysomes form, cells are HCR and RuBisCO is soluble (PubMed:8491708). Heterologous expression of 12 carboxysomal genes in E.coli (ccaA, ccmK2, ccmK3, ccmK4, ccmL, ccmM, ccmN, ccmO, ccmP, rbcL, rbcS, rbcX) leads to the formation of bodies that resemble carboxysomes, have densely packed paracrystalline arrays and RuBisCO activity. These structures open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery (PubMed:29922315). Large fluorescent tags can be attached simultaneously to both termini of this protein (adding about 60 kDa) and do not prevent carboxysome formation, showing proteins can be targeted to carboxysomes by fusion to endogenous proteins (PubMed:28963440). The M35 form (short form) is translated from the internal start codon Val-216 (gtg), under control of an internal Shine-Dalgarno sequence. Silent mutation of the codon for Val-216 to a non-start codon (i.e. gtg to gtc) leads to low level expression of the M35 form, and abnormal carboxysomes which contain the expected proteins, but does not grow in normal air. A strain that has a stop codon at residue 215 makes abnormal carboxysomes which contain the expected proteins, trace amounts of the N-terminus and is HCR, while a strain that expresses only M35 has only RuBisCO in the abnormal carboxysomes and is HCR. Belongs to the gamma-class carbonic anhydrase family.