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 (PubMed:20075914, PubMed:21765418). Required for optimal reconstitution of RuBisCO upon expression of rbcL-rbcS subunits in E.coli (PubMed:9171433). Homodimer. Interacts with the exposed C-terminal peptide of RbcL ('Glu-459-Asp-468'); binds 1 RbcL peptide per homodimer (PubMed:17574029, PubMed:20075914, PubMed:21765418). Contacts a second RbcL monomer via its peripheral polar surface (PubMed:21765418). A slightly longer RbcL peptide binds to RbcX2 with a higher affinity (PubMed:17574029). Most protein is cytoplasmic, but some is in the carboxysome. Transcribed in light but much less in the dark in both normal air and 1% CO(2); the nitrogen source has no effect on transcription. Constitutively expressed when grown on fructose. Part of the rbcL-rbcX-rbcS operon. 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.