One of shell proteins of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, ccbL-ccbS) is sequestered. Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome. The shell probably limits the diffusion of CO(2) into and out of the carboxysome. Homohexamer with a small central pore. Forms a CsoS2-CsoS1-RuBisCO complex. This bacterium makes alpha-type carboxysomes. Induced by growth in low levels of dissolved inorganic carbon (at protein level). The tight homohexamer forms a small pore which is positively charged. Belongs to the bacterial microcompartments protein family. CsoS1 subfamily.