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. When rbcL-rbcX-rbcS or rbcL-rbcS were overexpressed in E.coli no change in reconstituted RuBisCO activity was observed, which suggests RbcX plays no role in RuBisCO assembly in this system (PubMed:8472962). However in PubMed:8472962 E.coli chaperones groL and groS were also overexpressed, which may compensate for lack of rbcX (Probable). Homodimer. Interacts with the exposed C-terminal peptide of RbcL via its central cleft, contacts a second RbcL monomer via its peripheral polar surface (By similarity). RbcX and Raf1 can bind simultaneously to RbcL (PubMed:32451445). Most protein is cytoplasmic, but some is in the carboxysome. 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.