Required for alpha-carboxysome (Cb) assembly, mediates interaction between RuBisCO and the Cb shell (PubMed:25826651, PubMed:32123388) (Probable). The 3 C-terminal repeats act as the encapsulation signal to target proteins to the Cb; they are necessary and sufficient to target both CsoS2 and foreign proteins to the Cb (PubMed:33116131). The N-terminal repeats of this (probably) intrinsically disordered protein bind simultaneously to both subunits of RuBisCO; minimally 2 N-repeats are necessary for RuBisCO assembly into the Cb in vivo. Probably also interacts with the major shell proteins (CsoS1); that interaction would increase the local concentration of CsoS2 so that it can condense RuBisCO and full carboxysomes can be formed (PubMed:32123388). The long form is essential for Cb formation while the short form is not (PubMed:26608811). There are estimated to be 143 CsoS2A and 186 CsoS2B proteins per Cb (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. Probably interacts with the carboxysome (Cb) major shell protein CsoS1; this complex probably also interacts with RuBisCO (Probable). Interacts with CbbS (the small subunit of RuBisCO) but not the large subunit (CbbL) (PubMed:30305640). RuBisCO interacts with the N-terminal repeats of this protein; mutating the third and tenth basic residue of each N-repeat prevents binding. Binding is sensitive to ionic strength. A fusion of a single N-terminal repeat to the C-terminus of the large subunit of RuBisCO (cbbL) shows the repeat can lie between a CbbL dimer, making minor contacts to CbbS; thus each RuBisCO holoenzyme could bind 8 repeats. At least 2 N-repeats are required for RuBisCO assembly into the Cb (PubMed:32123388). Immunogold staining shows this is a shell protein (PubMed:10525740). C-terminally tagged protein immunoprecipitates whole carboxysomes, showing the C-terminus is on the exterior (PubMed:25826651). This bacterium makes alpha-type carboxysomes (Ref.4, PubMed:18258595). The production of the two protein products from this region is due to programmed -1 ribosomal frameshifting. In vivo about half the protein product is frameshifted. Has 3 domains; the N-terminal domain has 4 short repeats and binds RuBisCO. The central region has 6 longer repeats (Probable) (PubMed:32123388). The C-terminal domain has 3 repeats and a highly conserved C-terminal peptide (Probable). The 3 C-repeats serve as the encapsulation signal for the alpha-carboxysome (Cb), and are able to target foreign proteins to this organelle. Proteins can be targeted to the Cb by a single C-repeat, however more repeats yields more efficient targeting. The C-terminal peptide (CTP) is not required for cargo targeting and is probably on the outside of the Cb (PubMed:33116131, PubMed:25826651). Seen in gels as 2 forms (of 85 and 130 kDa, equal amounts of each) which have the same N-terminus, called respectively CsoS2A and CsoS2B (PubMed:10525740). Partial tryptic digestion and mass spectrometric analysis, as well as the presence of the shorter form even when a C-terminally tagged version is engineered, suggests CsoS2A is shorter at the C-terminus, but its sequence is not known (Probable). It has been shown these 2 forms are produced by ribosomal frameshifting (PubMed:26608811). Expressed in E.coli, CsoS2B (full-length) form. Expressed in E.coli, CsoS2A (short) form. Does not grow in air but does grow in 2% CO(2), called a high-CO(2) requiring phenotype, hcr. Cells do not make carboxysomes (PubMed:25826651). Also absolutely required for carboxysome formation in E.coli (PubMed:33116131). 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). Belongs to the CsoS2 family. Both Cso2A and Cso2B were originally thought to be glycosylated (PubMed:10525740). Later experiments do not show any evidence of glycosylation (PubMed:26608811). Truncated N-terminus.