One of the shell proteins of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome. The hexamer central pore probably regulates metabolite flux. The major shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Hexamers make sheets that form the facets of the polyhedral carboxysome (PubMed:22928045). The shell is 4.5 nm thick, as observed for CcmK hexamers (PubMed:28616951). Required for recruitment of CcmO to the pre-carboxysome (PubMed:22928045, PubMed:24267892). In PCC 7942 there are several CcmK paralogs with presumably functional differences; replacing the central pore residues (34-37) with those of either CcmK4 from this organism (Tyr-Met-Arg-Ala) or from an alpha-type carboxysome forming cyanobacterium (CsoS1 of P.marinus strain MIT 9313, Arg-Glu-Phe-Val) allows the bacterium to make carboxysomes, but the expression level is too low to know if the carboxysome is functional for CO(2) fixation (PubMed:25117559). 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. Homohexamer (PubMed:25117559, PubMed:30389783). Interacts with CcmO in the carboxysome (PubMed:22928045). Interacts with CcmN (PubMed:22461622). This cyanobacterium makes beta-type carboxysomes. The tight homohexamer forms a pore with an opening of about 5 Angstroms in diameter and is positively charged. Cells do not grow in normal air but do grow on 2% CO(2), called a high-CO(2) requiring phenotype, HCR (PubMed:8491708, PubMed:22928045). Cells make aberrantly large polar bodies instead of wild-type carboxysomes, no accumulation of CcmO (PubMed:22928045, PubMed:24267892). An alternatively generated deletion mutant does not form abnormal polar bodies (PubMed:25117559). 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. CcmK subfamily. A peculiar architecture - Issue 237 of June 2021