Biosynthetic pathways of two polypeptide subunits of the light-harvesting chlorophyll a/b protein complex

We have used an in vitro reconstitution system, consisting of cell-free translation products and intact chloroplasts, to investigate the pathway from synthesis to assembly of two polypeptide subunits of the light-harvesting chlorophyll-protein complex. These polypeptides, designated 15 and 16, are integral components of the thylakoid membranes, but they are products of cytoplasmic protein synthesis. Double immunodiffusion experiments reveal that the two polypeptides share common antigenic determinants and therefore are structurally related. Nevertheless, they are synthesized in vitro from distinct mRNAs to yield separate precursors, p15 and p16, each of which is 4,000 to 5,000 daltons larger than its mature form. In contrast to the hydrophobic mature polypeptides, the precursors are soluble in aqueous solutions. Along with other cytoplasmically synthesized precursors, p15 and p16 are imported into purified intact chloroplasts by a post-translational mechanism. The imported precursors are processed to the mature membrane polypeptides which are recovered exclusively in the thylakoids. The newly imported polypeptides are assembled correctly in the thylakoid lipid bilayer and they bind chlorophylls. Thus, these soluble membrane polypeptide precursors must move from the cytoplasm through the two chloroplast envelope membranes, the stroma, and finally insert into the thylakoid membranes, where they assemble with chlorophyll to form the light-harvesting chlorophyll protein complex.