Part of the carboxysome shell, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, cbbL-cbbS) is sequestered (Probable) (PubMed:22155772). It may control transport of RuBisCO reactants in and out of the carboxysome (Probable). There are estimated to be 6 CsoS1D hexamers per carboxysome (PubMed:22155772). Homotrimer. Forms a dimer of stacked trimers, the same faces interact (PubMed:19328811). A CsoS1-CsoS1D-CsoS2 complex can be isolated following expression in E.coli (PubMed:25826651). Fractionates with shell proteins (PubMed:22155772). This bacterium makes alpha-type carboxysomes (PubMed:19328811). 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. In each stacked hexamer one trimer forms a pore of about 14 Angstroms in diameter; open-closed and open-open trimers are seen in crystals. Glu-120 forms a salt bridge with Arg-121 of the adjacent subunit to close the pore. Dimerization of the trimers forms a channel-like compartment (volume 13,610 Angstroms(3)), 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.