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. A minor shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered (Probable). Hexamers form sheets that form the facets of the polyhedral carboxysome. The shell is 4.5 nm thick, as observed for CcmK proteins (PubMed:28616951). In PCC 7942 there are several CcmK paralogs with presumably functional differences; replacing the central pore residues (34-37) with those of CcmK2 from this organism (Tyr-Glu-Lys-Ile) allows the bacterium to make carboxysomes, but the expression level is too low to know if the carboxysome is functional for CO(2) fixation (Probable). This subunit probably makes both homohexamers and heterohexamers with CcmK3. The CcmK3-CcmK4 heterohexmers have been suggested to cap other hexamers, perhaps to alter metabolite flux (Probable). Homohexamer (PubMed:25117559). Interacts stably with CcmK3, probably forms heterohexamers with a 1:2 CcmK3:CcmK4 stoichiometry (PubMed:30389783) (Probable). This cyanobacterium makes beta-type carboxysomes (PubMed:25117559). Part of the carboxysome shell, not found in the interior (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 52 units of this protein per carboxysome, the numbers are stable under low light and high light, and increase under high CO(2) (at protein level). The CcmK3:CcmK4 ratio of 1:3.8 is stable over all growth conditions. The tight homohexamer forms a pore with an opening of about 5 Angstroms in diameter and is positively charged. Single deletion has a wild-type phenotype, a double ccmK3-ccmK4 deletion has slightly larger, aggregated instead of spatially separated carboxysomes, and has a photosynthetic affinity for inorganic carbon between wild-type and carboxysome-less strains (PubMed:22928045). In another study single mutant grows slower in air and low light, double ccmK3-ccmK4 mutant has similarly impaired growth rate; ccmK3 does not complement the double deletion, while ccmK4 does (PubMed:30389783). 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. The absence of ccaA, ccmK3, ccmK4, ccmP and rbcX leads to less active RuBisCO. Belongs to the bacterial microcompartments protein family. CcmK subfamily.