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. Homohexamer with a small central pore. Forms a CsoS2-CsoS1-RuBisCO complex. This bacterium makes alpha-type carboxysomes. Transcribed during light cycle, reaching a peak after 6 hours of illumination, mRNA becomes undetectable during the dark. Part of the csoS1-ccbL-ccbS operon. The tight homohexamer forms a small pore which is positively charged. Belongs to the bacterial microcompartments protein family. CsoS1 subfamily.