Part of the carboxysome shell, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, cbbL-cbbS) is sequestered. It may control transport of RuBisCO reactants in and out of the carboxysome. Homotrimer. Forms a dimer of stacked trimers, the same faces interact. Probably forms a CsoS1-CsoS1D-CsoS2 complex. This bacterium makes alpha-type carboxysomes. Contains 2 BMC domains, trimerizes in a staggered manner to give a hexamer; each subunit in one trimer interacts with 2 subunits in the facing trimer. Each stacked hexamer can form a pore of about 14 Angstroms in diameter. Dimerization of the trimers forms a channel-like compartment, accessible via an open pore. This channel may be large enough to accomodate transport of substrates into and out of the carboxysome. Belongs to the EutL/PduB family.