Required for carboxysome formation; the N-terminus interacts with CcmM which itself binds RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS), while the C-terminal 18 residues interact with carboxysome shell protein CcmK2 (PubMed:22461622). Required for growth in normal air (PubMed:2509426). 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. Interacts with CcmM via the N-terminus of CcmN. Interacts with CcmK2 via the 18 C-terminal residues. This cyanobacterium makes beta-type carboxysomes. Carboxysome size and components vary with growth conditions. When grown in ambient air at medium light (50 uE meter(-2) second(-1)) there are 74 units of this protein per carboxysome, the numbers decrease under low light and high CO(2), and increase under high light (at protein level). The N-terminus interacts with CcmM, while the 18 C-terminal residues (encapsulation peptide) are required for interaction with the hexameric shell proteins (CcmK2) and are predicted to form a helix. The 2 regions are separated by a linker. Cells with a disrupted or mutated protein do not have carboxysomes, instead have less dense inclusions, do not grow in normal air (PubMed:2509426). Targeted disruption do not grow normally in air but have wild-type growth in 3% CO(2), called a high-CO(2) requiring phenotype, HCR. Cells have a single polar aggregate instead of carboxysomes; RuBisCO localizes to polar spots consistent with the polar aggregates (PubMed:22461622, PubMed:24267892). 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 CcmN family.