The major shell protein 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 (By similarity). There are estimated to be 538 CsoS1 hexamers per carboxysome; note this number includes the probable carboxysome shell vertex proteins CsoS4A and CsoS4B (PubMed:22155772). Homohexamer with a small central pore (By similarity). A CsoS1-CsoS1D-CsoS2 complex can be isolated following expression in E.coli (PubMed:25826651). Forms a CsoS2-CsoS1-RuBisCO complex (By similarity). This bacterium makes alpha-type carboxysomes. The tight homohexamer forms a small pore which is positively charged. Belongs to the bacterial microcompartments protein family. CsoS1 subfamily. Truncated N-terminus.