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. Homohexamer. Interacts with CcmN and CcmO in the carboxysome. This cyanobacterium makes beta-type carboxysomes. The tight homohexamer forms a small pore which is positively charged. Belongs to the bacterial microcompartments protein family. CcmK subfamily.