Required for formation of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Required for recruitment of major shell protein CcmK2 to the pre-carboxysome. Suggested to be a carboxysome shell protein. Homooligomerizes, possibly as a trimer, interacts with CcmK in the carboxysome. This cyanobacterium makes beta-type carboxysomes. Has 2 BMC domains, is thought to trimerize giving a hexamer that may interact with CcmK proteins in the carboxysome shell. Belongs to the bacterial microcompartments protein family.