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:23949415, PubMed:24504539). May interact with CcmK2, this occurs at very high CcmK2 concentrations (Probable). Interacts with full-length CcmM (By similarity). This cyanobacterium makes beta-type carboxysomes (Probable). Probably forms vertices in the carboxysome (Probable). The tight homopentamer forms a pore with an opening of 4-5 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.