Probably part of the carboxysome shell, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. It is thought that this protein controls transport of RuBisCO reactants in and out of the carboxysome; residual densities in the 4 X-ray structures suggest that differing compounds bind in interior pockets, depending on the open or closed state of the pore. A dimer of stacked trimers, the same faces interact. This cyanobacterium makes beta-type carboxysomes. Contains 2 BMC domains, trimerizes to give a hexamer. Each trimer forms a pore with an opening of about 13 Angstroms in diameter; depending on the conformation of conserved residues Glu-69 and Arg-70 the pore is open or closed. Dimerization of the trimers forms a small barrel-like compartment, accessible via the pore. Barrels with one open and one closed pore or with 2 closed pores have been seen. A pocket that is the right size to loosely bind RuBisCO substrates and products is found between the 2 BMC domains in each monomer. Tagging the C-terminus of this protein with a fluroscent tag completely disrupts carboxysome formation. Heterologous expression of 12 carboxysomal genes in E.coli (ccaA, ccmK2, ccmK3, ccmK4, ccmL, ccmM, ccmN, ccmO, ccmP, rbcL, rbcS, rbcX) leads to the formation of bodies that resemble carboxysomes, have densely packed paracrystalline arrays and RuBisCO activity. These structures open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery. The absence of ccaA, ccmK3, ccmK4, ccmP and rbcX leads to less active RuBisCO. Belongs to the EutL/PduB family.