Probably forms vertices in the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. Has been modeled to induce curvature upon insertion into an otherwise flat hexagonal molecular layer of CcmK subunits. Homopentamer (PubMed:24504539). Interacts with full-length CcmM (By similarity). This cyanobacterium makes beta-type carboxysomes. Probably forms vertices in the carboxysome. The tight homopentamer forms a pore with an opening of about 5 Angstroms in diameter which opens into a wider tunnel at the base of the truncated pyramid. The pore is positively charged. Belongs to the CcmL/EutN family. CcmL subfamily.