Probably forms vertices in the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, cbbL-cbbS) is sequestered. Has been modeled to induce curvature upon insertion into an otherwise flat hexagonal layer of major carboxysome subunits. Homopentamer. This bacterium makes alpha-type carboxysomes. Induced by growth in low levels of dissolved inorganic carbon (at protein level). The tight homopentamer forms a pore with an opening of about 3.5 Angstroms in diameter which is positively charged. Belongs to the CcmL/EutN family. CsoS4 subfamily.