Characterization of the Lil3 protein during deetiolation of Hordeum vulgare

Abstract

Angiosperm plants growing in darkness are free of chlorophyll and develop etioplasts instead of chloroplasts the characteristic organelles of green plants. Upon illumination, chlorophyll and the chlorophyll binding proteins of the photosynthetic machinery rapidly accumulate. Recently, chlorophyll has been shown to bind to Lil3 immediately after illumination of etioplasts. Lil3 is a light harvesting like proteins that shares an alpha helix motif with the light harvesting proteins of the photosystem complexes. However, it is unknown how much of the Lil3 protein complex is present during development of the photosystem complexes. Here, we show that Lil3 is a membrane protein and that its amount is constant throughout deetiolation of dark grown barley seedlings. We found that the Lil3 protein immediately assembles into two protein complexes upon the onset of illumination of barley leaves, but is not present as a protein complex in etioplasts. In contrast, equal amounts of the protein were found in etioplasts and in any of the developmental time points during biogenesis of chloroplasts. Our results demonstrate that the Lil3 proteins could provide the missing link for transfer of the chlorophyll free membrane of etioplasts into the chlorophyll rich membrane of chloroplasts. The Lil3 protein could operate as chlorophyll storage and integrate into an enzymatic chlorophyll delivery chain to enable the assembly of the photosynthetic machinery in the membrane of plant plastids. This report is the starting point for a more detailed characterization of the Lil3 function. The next level of further investigations will be directed to understand the composition, folding, and structure of the Lil3 protein complexes.