An RbcL-specific chaperone. The central cleft of the RbcX homodimer (RbcX2) binds the C-terminus of an RbcL monomer, stabilizing the C-terminus and probably preventing its reassociation with chaperonin GroEL-ES. At the same time the peripheral region of RbcX2 binds a second RbcL monomer, bridging the RbcL homodimers in the correct orientation. The RbcX2(2)-bound RbcL dimers then assemble into the RbcL8 core (RbcL8-(RbcX2)8). RbcS binding triggers the release of RbcX2. Homodimer (Probable). Interacts with the exposed C-terminal peptide of endogenous RbcL ('Lys-460-Asp-470') via its central cleft, as well as C-terminal peptides from other cyanobacterial RbcL (PubMed:20075914, PubMed:21765418). Contacts a second RbcL monomer via its peripheral polar surface (By similarity). Most protein is cytoplasmic, but some is in the carboxysome. This cyanobacterium makes beta-type carboxysomes (Probable). The homodimer has 2 functional domains, a central cleft essential for production of soluble RbcL in which the RbcL peptide binds, and a polar surface which plays a role in correct RbcL subunit arrangement. Belongs to the RbcX family.