The form I Rubisco of autotrophic bacteria, algae and plants is a complex of eight large (RbcL) and eight small (RbcS) subunits. It fixes atmospheric CO2 in the dark reaction of photosynthesis. As shown for the cyanobacterial enzyme, folding of the RbcL subunits is mediated by the GroEL–GroES chaperonin system, and assembly requires the specialized chaperone RbcX, a homodimer of ~15-kDa subunits. Here we present the 3.2-Å crystal structure of a Rubisco assembly intermediate, consisting of the RbcL8 core with eight RbcX2 molecules bound. The structure reveals the molecular mechanism by which RbcX2 mediates oligomeric assembly. Specifically, RbcX2 provides positional information for proper formation of antiparallel RbcL dimers, thereby preventing RbcL–RbcL misalignment and off-pathway aggregation. The RbcL8(RbcX2)8 structure also suggests that RbcS functions by stabilizing the '60s loop' of RbcL in the catalytically active conformation.
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We thank S. Saschenbrecker for performing initial crystallization trials and the Joint Structural Biology Group staff at the European Molecular Biology Laboratory and European Synchrotron Radiation Facility, Grenoble, France, for their support at beamlines ID29, ID23-2 and ID23-1.
The authors declare no competing financial interests.
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Bracher, A., Starling-Windhof, A., Hartl, F. et al. Crystal structure of a chaperone-bound assembly intermediate of form I Rubisco. Nat Struct Mol Biol 18, 875–880 (2011). https://doi.org/10.1038/nsmb.2090
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