Required for alpha-carboxysome (Cb) assembly, mediates interaction between RuBisCO and the Cb shell. The protein is probably highly flexible (Probable). The 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. The N-terminal repeats of this protein bind simultaneously to both subunits of RuBisCO. 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 (By similarity). There are estimated to be 163 CsoS2 proteins per carboxysome; unlike H.neapolitanus only 1 form is seen (PubMed:22155772). Probably interacts with the carboxysome major shell protein CsoS1 via the N-terminal domain (Probable). A CsoS1-CsoS1D-CsoS2 complex can be isolated following expression in E.coli (PubMed:25826651). Interacts via its N-terminal repeats with RuBisCO (By similarity). This bacterium makes alpha-type carboxysomes. Has 3 domains; the N-terminal domain has 4 short repeats, the central region has 6 longer repeats (Probable). The C-terminal domain has 2 repeats and a highly conserved C-terminal peptide. The C-repeats serve as the encapsulation signal for the alpha-carboxysome, and are able to target foreign proteins to this organelle (By similarity). Unlike H.neapolitanus and predictions for P.marinus strain MIT 9313, this protein is not thought to have ribosomal frameshifting. Belongs to the CsoS2 family.