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 (By similarity). In an E.coli expression system not absolutely necessary, its presence leads to fewer defective carboxysomes, suggesting this subunit may play a role in assembly (PubMed:22184212). Unlike beta-carboxysomes, alpha-carboxysomes (Cb) can form without cargo protein. CsoS2 is essential for Cb formation and is also capable of targeting foreign proteins to the Cb. The Cb shell assembles with the aid of CsoS2; CsoS1A, CsoS1B and CsoS1C form the majority of the shell while CsoS4A and CsoS4B form vertices. CsoS1D forms pseudohexamers that probably control metabolite flux into and out of the shell. 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. Not required for growth in ambient air. Expression of 10 genes for alpha-carboxysome (Cb) proteins (cbbL-cbbS-csoS2-csoS3-csoS4A-csoS4B-csoS1C-csoS1A-csoS1B-csoS1D) in E.coli generates compartments that resemble Cb, contain RuBisCO and have its catalytic activity, showing it is possible to make artificial, functional Cb using these 10 genes. Cargo proteins can be targeted to these organelles (PubMed:22184212). Artificial Cb assembly in E.coli requires csoS2-csoS4A-csoS4B-csoS1C-csoS1A-csoS1B-csoS1D (but not the gene for carbonic anhydrase, csoS3). Targeting proteins to the organelle requires at least one of the CsoS2 C-repeats; 3 repeats gives the best localization. A nanoreactor of the Cb shell proteins has been engineered which generates H(2) using a ferredoxin-hydrogenase fusion (AC P07839-Q9FYU1) and a flavodoxin/ferredoxin--NADP reductase (AC A0A0K3QZA5) targeted separately to the Cb; the hydrogenase has first to be matured and activated by HydGXEF (AC Q8EAH9, Q8EAH8, Q8EAH7 and Q8EAH6 respectively). Encapsulation increases H(2) production about 20% during anaerobic growth, and over 4-fold more during aerobic growth (PubMed:33116131). Belongs to the EutL/PduB family.