A probably 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 7418 there are several CcmK paralogs with presumably functional differences. This subunit probably only makes heterohexamers with CcmK4. Heterohexamers can also make dodecamers, formation depends on buffer conditions. Interacts stably with CcmK4, forming heterohexamers that can make dodecamers. Heterohexamers have a 1:2 CcmK3:CcmK4 stoichiometry. Upon expression in E.coli forms oligomers that could be dimers or trimers, but never hexamers; bulky residues in the pore region probably preclude the formation of homohexamers. This cyanobacterium makes beta-type carboxysomes. Belongs to the bacterial microcompartments protein family. CcmK subfamily.