title: Structure and function of RbcX, an assembly chaperone for hexadecameric Rubisco.
abstract: After folding, many proteins must assemble into oligomeric complexes to become biologically active. Here we describe the role of RbcX as an assembly chaperone of ribulose-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme responsible for the fixation of atmospheric carbon dioxide. In cyanobacteria and plants, Rubisco is an approximately 520 kDa complex composed of eight large subunits (RbcL) and eight small subunits (RbcS). We found that cyanobacterial RbcX functions downstream of chaperonin-mediated RbcL folding in promoting the formation of RbcL(8) core complexes. Structural analysis revealed that the 15 kDa RbcX forms a homodimer with two cooperating RbcL-binding regions. A central cleft specifically binds the exposed C-terminal peptide of RbcL subunits, enabling a peripheral surface of RbcX to mediate RbcL(8) assembly. Due to the dynamic nature of these interactions, RbcX is readily displaced from RbcL(8) complexes by RbcS, producing the active enzyme. The strategies employed by RbcX in achieving substrate specificity and efficient product release may be generally relevant in assisted assembly reactions.
authors.affiliation: Department of Cellular Biochemistry, Max Planck Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany.
authors.forename: Sandra
authors.initials: S
authors.lastname: Saschenbrecker
authors.forename: Andreas
authors.initials: A
authors.lastname: Bracher
authors.forename: Karnam Vasudeva
authors.initials: KV
authors.lastname: Rao
authors.forename: Bharathi Vasudeva
authors.initials: BV
authors.lastname: Rao
authors.forename: F Ulrich
authors.initials: FU
authors.lastname: Hartl
authors.forename: Manajit
authors.initials: M
authors.lastname: Hayer-Hartl
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000502:physiology*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D018832:Molecular Chaperones / Q000737:chemistry* / Q000502:physiology*; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D009154:Mutation; D010455:Peptides / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D012273:Ribulose-Bisphosphate Carboxylase / Q000737:chemistry* / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D018832:Molecular Chaperones; D010455:Peptides; C522129:RbcX protein, cyanobacteria; C101625:RbcL protein, plastid; D012273:Ribulose-Bisphosphate Carboxylase