RAF2 is a Rubisco Assembly Factor in Arabidopsis thaliana

The assembly of the carbon fixing enzyme Rubisco is a complex process and involves many chaperones including RAF1, RbcX and the chaperonins. Recent evidence pointed to the existence of another chaperone, RAF2, conserved among photosynthetic organisms. The protein has similarity to pterin-4α-carbinolamine dehydratase (PCD), although appears to lack activity in the species studied to date. Fristedt et al. assessed the influence of RAF2 on Rubisco assembly in Arabidopsis thaliana. Using an anti-RAF2 antibody, the authors showed that RAF2 accumulates prior to Rubisco during greening and is largely confined to the chloroplast stroma. raf2 mutants were slower-growing and appear paler than wild-type (WT) plants, an effect exacerbated in short days. The mutants demonstrated normal formation of thylakoid membranes and had WT levels of proteins constituting the photosynthetic electron transport chain. However, there was a dramatic reduction in the level of Rubisco in raf2 mutants, which led to reduced carbon assimilation at irradiances above 200 μmol m^-2 s^-1.  Formation of aggregates of Rubisco large subunits in raf2 seedlings suggests RAF2 may be involved in chaperoning small subunits to/into large subunit complexes. (Summary by Mike Page) Plant J. 10.1111/tpj.13849