Probably forms vertices in the carboxysome. Has been modeled to induce curvature upon insertion into an otherwise flat hexagonal layer of major carboxysome subunits (Probable). A minor shell protein, only 12 pentamers of CsoS4A/CsoS4B are calculated to be present in each carboxysome. The 2 CsoS4 proteins contribute to the impermeability of the carboxysome to CO(2) (PubMed:19844578). Its central pore is probably too small to allow passage of metabolites; its function might be to anchor different proteins or metabolites to the carboxysome (Probable). 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. Homopentamer. Probably forms vertices in the icosahedral carboxysome (By similarity). This bacterium makes alpha-type carboxysomes (PubMed:19844578). Forms a stable homopentamer with a convex and concave side. A short loop (residues 60-69) protrudes above the center of the convex face to different degrees. The tight homopentamer forms a pore with an opening of about 2.9 Angstroms in diameter at its most narrow, surrounded by the amino acids Ser-Gly-Ser-Ala-Ala (one from each subunit). A double csoS4a-csoS4B disruption does not grow in ambient air, has a wild-type growth rate in 5% CO(2), called a high-CO(2) requiring phenotype, hcr. It does not grow to the same cell density as wild-type. 1/3 fewer carboxysomes are seen per cell, about 13% of which are elongated. The carboxysome shell is more permeable than usual to inorganic carbon. 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). 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 CcmL/EutN family. CsoS4 subfamily.