Probably part of the carboxysome shell, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. It is thought that this protein controls transport of RuBisCO reactants in and out of the carboxysome. A dimer of stacked trimers, the same faces interact. This cyanobacterium makes beta-type carboxysomes. Contains 2 BMC domains, trimerizes to give a hexamer. Each trimer forms a pore with an opening of about 13 Angstroms in diameter; depending on the conformation of conserved residues Glu-69 and Arg-70 the pore is open or closed. Dimerization of the trimers forms a small barrel-like compartment, accessible via the pore. Belongs to the EutL/PduB family.