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. Probably the major shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. The central pore probably regulates metabolite flux (PubMed:18292340). Hexamers make sheets that form the facets of the carboxysome (Probable) (PubMed:19844993). Homohexamer (PubMed:19844993, PubMed:18292340, PubMed:19177356, PubMed:24419393, PubMed:31603944). Interacts with full-length CcmM (PubMed:17993516). Forms mixed heterohexamers of all possible stoichiometries with CcmK2, which might form dodecamers. Only very weak interactions with CcmK3 and CcmK4 were seen (PubMed:31603944). Interacts with CcmN and CcmO in the carboxysome (By similarity). This cyanobacterium makes beta-type carboxysomes (Probable). Forms walls in the polyhedral carboxysome (Probable). The tight homohexamer forms a pore with an opening of about 5.0 Angstroms in diameter which is positively charged. Sulfate ions are bound in one crystal form in both the pore and between hexamers which may represent metabolite flux. 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.