Probably forms vertices in the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Has been modeled to induce curvature upon insertion into an otherwise flat hexagonal molecular layer of CcmK subunits. 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. Homopentamer (Probable). Interacts with full-length CcmM (By similarity). This cyanobacterium makes beta-type carboxysomes (PubMed:24267892). Probably forms vertices in the polyhedral carboxysome (Probable). Carboxysome size and components vary with growth conditions. When grown in ambient air at medium light (50 uE meter(-2) second(-1)) there are 7.4 units of this protein per carboxysome, the numbers are stable under low light, and increase under high light and high CO(2) (at protein level). The tight homopentamer forms a pore with an opening of about 5 Angstroms in diameter which opens into a wider tunnel at the base of the truncated pyramid. The pore is positively charged. An insertion in this gene leads to rod-shaped carboxysomes and an inability to grow in normal air, called a high-CO(2) requiring phenotype, HCR. When ccmL-ccmM-ccmN-ccmO are deleted no carboxysomes form, cells are HCR and RuBisCO is soluble. Both mutants grow on 2% CO(2). 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 CcmL/EutN family. CcmL subfamily.