An RbcL-specific chaperone. Required for assembly of the RbcL8 core, acting downstream of the major chaperonin (GroEL-GroES). Acts on newly folded RbcL, has a transient dynamic interaction with RbcL and is eventually displaced by RbcS (PubMed:17574029). 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 (By similarity). Required for optimal reconstitution of RuBisCO into its RbcL8S8 holoenzyme form upon expression of rbcL-rbcS subunits in E.coli, and probably also in situ. A frameshift mutation that replaces half the protein reduces accumulation of both RbcL and RbcS subunits and halves activity of RuBisCO in situ and in E.coli (PubMed:15564522). Homodimer (RbcX2) (PubMed:17574029, Ref.6). Interacts with the exposed C-terminal peptide of RbcL ('Glu-459-Asp-468'); binds 2 RbcL peptides per RbcX2, stapling them into an RbcL2 dimer. A slightly longer peptide binds with a higher affinity, but no long-term stable interaction with RbcL is detected (PubMed:17574029, PubMed:20075914). 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). 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.