LIL3, a light-harvesting-like protein, plays an essential role in chlorophyll and tocopherol biosynthesis

Abstract

The light-harvesting chlorophyll binding (LHC) proteins are major constituents of the eukaryotic photosynthetic machinery. In plants, six different groups of proteins, LHC-like proteins, share a conserved motif with LHC. Although the evolution of LHC and LHC-like proteins is proposed to be a key for the diversification of modern photosynthetic eukaryotes, our knowledge on the evolution and functions of LHC-like proteins are still limited. In this study, we aimed to specifically understand the function of one type of LHC-like proteins, LIL3 proteins, by analyzing Arabidopsis mutants lacking these proteins. The Arabidopsis genome contains two gene copies for LIL3, LIL3:1 and LIL3:2. In the lil3:1/lil3:2 double mutant the majority of chlorophyll molecules is conjugated with an unsaturated geranylgeraniol side chain. This mutant is also deficient in α-tocopherol. These results indicate that reduction of both the geranylgeranyl side chain of chlorophyll and geranylgeranyl pyrophosphate, which is also an essential intermediate of tocopherol biosynthesis, is compromised in the lil3 mutants. We found that the content of geranylgeranyl reductase responsible for these reactions was severely reduced in the lil3 double mutant, while the mRNA level for this enzyme was not significantly changed. We demonstrated an interaction of geranylgeranyl reductase with both LIL3 isoforms by using a split ubiquitin assay, bimolecular fluorescence complementation, and combined blue-native and SDS polyacrylamide gel electrophoresis. Collectively, we propose that LIL3 is functionally involved in chlorophyll and tocopherol biosynthesis by stabilizing geranylgeranyl reductase