One of the 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. Homohexamer with a small central pore (PubMed:25117559). Forms a CsoS2-CsoS1-RuBisCO complex (Probable). This cyanobacterium makes alpha-type carboxysomes, but upon expression in Synechococcus elongatus PCC 7942 this protein seems to be incorporated into beta-type carboxysomes. The tight homohexamer forms a pore with an opening of about 5 Angstroms in diameter. Belongs to the bacterial microcompartments protein family. CsoS1 subfamily.