Structure and mechanism of the Rubisco-assembly chaperone Raf1

Biogenesis of the photosynthetic enzyme Rubisco, a complex of eight large (RbcL) and eight small (RbcS) subunits, requires assembly chaperones. Here we analyzed the role of Rubisco accumulation factor1 (Raf1), a dimer of similar to 40-kDa subunits. We find that Raf1 from Synechococcus elongatus acts downstream of chaperonin-assisted RbcL folding by stabilizing RbcL antiparallel dimers for assembly into RbcL(8) complexes with four Raf1 dimers bound. Raf1 displacement by RbcS results in holoenzyme formation. Crystal structures show that Raf1 from Arabidopsis thaliana consists of a beta-sheet dimerization domain and a flexibly linked alpha-helical domain. Chemical cross-linking and EM reconstruction indicate that the beta-domains bind along the equator of each RbcL(2) unit, and the alpha-helical domains embrace the top and bottom edges of RbcL(2). Raf1 fulfills a role similar to that of the assembly chaperone RbcX, thus suggesting that functionally redundant factors ensure efficient Rubisco biogenesis.