Required for formation of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Required for recruitment of major shell protein CcmK2 to the pre-carboxysome (PubMed:22928045, PubMed:24267892). Suggested to be a carboxysome shell protein, but it is not detected in gels, mass spectrometry or by protein sequencing (Probable). 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 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. Homooligomerizes, possibly as a trimer, interacts with CcmK2 in the carboxysome. This cyanobacterium makes beta-type carboxysomes. Has 2 BMC domains, is thought to trimerize giving a hexamer that may interact with CcmK proteins in the carboxysome shell. Cells make aberrantly large polar bodies instead of wild-type carboxysomes, do not grow in normal air but do grow on 4-5% CO(2), called a high-CO(2) requiring phenotype, HCR, no accumulation of CcmK2 (PubMed:8491708, PubMed:22928045, PubMed:24267892). 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. Belongs to the bacterial microcompartments protein family.