The Rebeiz Foundation for Basic Research (RFFBR) Paper Award for 2010 has been awarded to Dr. Manajit Hayer-Hartl and her colleagues: Cuimini Liu, Anna L. Young, Amanda Sarling-Windho, Andreas, Bracher, Sandra Saschenbrecher, Bharati Vasudeva Rao, Karnam Vasudeva Rao, Otto BerninghausenThorsten Mielke, F. Ulrich Hartl, and Roland Beckmann, for their paper entitled " Coupled Chaperone action in folding and assembly of hexameric Rubisco”. The paper appeared in Nature, 463: 197-204(2010).
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is one of the most abundant enzymes in nature. It is central to the photosynthetic process in which atmospheric carbon-dioxide is fixed into organic compounds, and thus has important implications in improving crop yield and controlling greenhouse gas-induced climate change. In plants and cyanobacteria form I Rubisco has a complex oligomeric structure consisting of 8 large (RbcL) and 8 small (RbcS) subunits. The RbcL subunits arrange as a tetramer of catalytic anti-parallel dimers, capped by 4 RbcS subunits at the top and bottom, respectively. Rubisco catalyses two opposite reactions: photosynthetic carbon fixation (with CO2 as the substrate) and photorespiration (with O2 as the substrate). Photorespiration is a competing reaction that decreases the efficiency of carbon fixation. Extensive efforts have focused on re-engineering a more efficient Rubisco enzyme, but these efforts have so far been thwarted by the limited understanding of the folding and assembly pathway of form I Rubisco and by the resulting inability to reconstitute the enzyme in vitro. In contrast, the homologous form II Rubisco has been reconstituted with the bacterial chaperonin, GroEL, and its co-factor GroES. This suggested the involvement of an additional factor(s) in the assembly of form I Rubisco. Indeed, recently the authors showed that the chaperone RbcX acts downstream of GroEL/GroES in promoting the assembly of cyanobacterial form I RbcL subunits upon co-expression in E. coli. RbcX, a ~15 kDa protein, functions as an arc-shaped homodimer (RbcX2) that binds the conserved C-terminal sequence of RbcL in a central hydrophobic cleft. A peripheral hydrophilic region of RbcX2 is also necessary for the assembly process.In this study the authors have analyzed the formation of cyanobacterial form I Rubisco by in vitro reconstitution and cryo-electron microscopy. They show that RbcL subunit folding by the GroEL/GroES chaperonin is tightly coupled with assembly mediated by RbcX2. RbcL monomers remain partially unstable and retain high affinity for GroEL until captured by RbcX2. As revealed by the structure of an RbcL8-(RbcX2)8 assembly intermediate, RbcX2 acts as a molecular staple in stabilizing the RbcL subunits as dimers and facilitates RbcL8 core assembly. Finally, addition of RbcS results in a conformational change in the RbcL subunits and the release of RbcX2. These experiments provide critical insight into the mechanism of Rubisco holoenzyme formation and open the way to renewed attempts at improving the agriculturally relevant Rubisco from plants. It is suggested that specific assembly chaperones may be required more generally in the formation of complex oligomeric structures when folding is closely coupled to assembly.
In selecting this paper for the fifth RFFBR Paper Award, the Board of directors of the Rebeiz Foundation felt that the described work will greatly benefit other chloroplast researchers, and enhance our knowledge of the relationship of photosynthesis to the molecular bioengineering of a more efficient Rubisco.
A copy of the certificate of recognition is given below
From left to right:Bharati v. Rao; Kausik Chakraborty; Karmam V. Rao; Alex Hastie; Cuimin Liu; Florian Georgescauld; Oliver Mueller-Cajar; Taotao Chen; Manajit Hayer-Hartl; F. Ulrich Hartl; Jyoti Sinha;Eva-Maria Weiland; Manal Chatila; Giulia Calloni; Amanda Starling-Windhof; Nadine Wischnewski; Sonia M. Schermann