One of shell proteins of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, ccbL-ccbS) is sequestered. Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome (By similarity). The shell probably limits the diffusion of CO(2) into and out of the carboxysome (Probable). There are estimated to be 540 CsoS1B proteins per carboxysome (Ref.4). 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. Homohexamer with a small central pore (By similarity). Interacts with the N-terminus (residues 1-136) of RuBisCO (CbbL) (PubMed:30305640). This bacterium makes alpha-type carboxysomes. The tight homohexamer forms a small pore which is positively charged. 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, PubMed:30305640). 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 bacterial microcompartments protein family. CsoS1 subfamily.