A non-essential, minor shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Hexamers form sheets that form the facets of the polyhedral carboxysome. In PCC 7942 there are several CcmK paralogs with presumably functional differences (PubMed:22928045, PubMed:25117559). This subunit probably only makes heterohexamers with CcmK4. The CcmK3-CcmK4 heterohexmers have been suggested to cap other hexamers, perhaps to alter metabolite flux (Probable). Interacts stably with CcmK4, probably forms heterohexamers with a 1:2 CcmK3:CcmK4 stoichiometry (PubMed:30389783) (Probable). Bulky residues in the pore region probably preclude the formation of homohexamers by this subunit (Probable). 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 15 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. 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 is wild-type, double ccmK3-ccmK4 mutant has 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.