Probably the major shell protein 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. Stacked hexamers, with the concave faces together, have also been crystallized. Interacts preferentially with itself, then with CcmK1 and CcmK4a in vitro (PubMed:22748766). May interact with CcmL, this occurs at very high CcmK2 concentrations (Probable). Interacts with CcmN and CcmO in the carboxysome (By similarity). This cyanobacterium makes beta-type carboxysomes. The tight homhexamer forms an irregular pore with an opening about 4.8 X 6.5 Angstroms. Belongs to the bacterial microcompartments protein family. CcmK subfamily.