One of the shell proteins of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. The central pore probably regulates metabolite flux (PubMed:16081736). Hexamers make sheets that form the facets of the polyhedral carboxysome (Probable) (PubMed:19844993). Homohexamer, might also make dodecamers (PubMed:16081736, PubMed:19177356, PubMed:31603944). Interacts with full-length CcmM (PubMed:17993516). Forms mixed heterohexamers of all possible stoichiometries with CcmK1, which might form dodecamers. Only very weak interactions with CcmK3 and CcmK4 were seen (PubMed:31603944). This cyanobacterium makes beta-type carboxysomes. The tight homohexamer forms a pore with an opening of about 5.9 Angstroms in diameter which is positively charged. The C-terminal flexible tail, which is the most variable part of CcmK proteins, seems to be involved in packing of hexamers in layers. Belongs to the bacterial microcompartments protein family. CcmK subfamily.