The major shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, ccbL-ccbS) is sequestered (PubMed:7934888, PubMed:16535117). Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome (PubMed:17518518). The shell probably limits the diffusion of CO(2) into and out of the carboxysome (Probable). Molecular modeling shows the central pore of this protein is selectively permeable to anions such as HCO(3) rather than CO(2) or O(2) (Probable). There are estimated to be 2970 CsoS1A/CsoS1C proteins per carboxysome (the proteins differ by only 1 residue) (Ref.5). 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; the concave side is mostly positive electrostatic potential, whereas the convex side is mostly negative electrostatic potential (PubMed:17518518). Forms a CsoS2-CsoS1-RuBisCO complex (Probable). Interacts with the N-terminus (residues 1-136) of RuBisCO (CbbL) (PubMed:30305640). This bacterium makes alpha-type carboxysomes. The tight homohexamer forms a pore with an opening of about 4 Angstroms in diameter which is positively charged. Does not grow in ambient air, has a wild-type growth rate in 5% CO(2), called a high-CO(2) requiring phenotype, hcr. Fewer carboxysomes are present, RuBisCO is found to be soluble rather than in the carboxysome. Required for growth in ambient air (PubMed:31406332). 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). When this gene is expressed in S.elongatus PCC 7942 it colocalizes with its beta-carboxysomes, suggesting there a range of modular possibilites for the carboxysome constituent proteins. These experiments open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery. Belongs to the bacterial microcompartments protein family. CsoS1 subfamily.