Lil3 and the protein’s function in chlorophyll synthesis.

The family of light harvesting like proteins contain proteins with one, two, and three helices. The two helix light harvesting-like protein 3 (Lil3) has been reported to regulate Chl and tocopherol biosynthesis in Arabidopsis thaliana by stabilizing Geranylgeranioloxidoreductase (GGR) against degradation. The Lil3 dependent regulation remained however unclear. We find that Lil3 assembles into three distinct fluorescent membrane protein complexes containing chlorophyll synthase (CHS), protochlorophyllide oxidoreductase (POR) and GGR. A direct interaction of Lil3 with CHS and POR and assembly of Lil3 as homo- and hetero- dimers is shown. We further show that Lil3 binds Chl and that Lil3 dimerization may precede Chl binding to Lil3. No direct interaction between GGR and Lil3 was found. However, direct interaction of GGR with POR and CHS indicates that structural stabilization of GGR is compromised in the absence of Lil3 dependent coordination of the GGR binding partners.