Synthesis of thylakoid membrane proteins by chloroplasts isolated from spinach: Cytochrome b559 and p700-chlorophyll a-protein

Intact chloroplasts, purified from spinach leaves by sedimentation in density gradients of colloidal silica, incorporate labeled amino acids into at least 16 different polypeptides of the thylakoid membranes, using light as the only source of energy. The thylakoid products of chloroplast translation were visualized by subjecting membranes purified from chloroplasts labeled with [88S]methionine to electrophoresis in high-resolution, SDS-containing acrylamide gradient slab gels and autoradiography. The apparent mol wt of the labeled products ranged from ≤10,000 to >70,000. One of the labeled products is the apoprotein of the P700-chlorophyll a-protein (CPI). The CPI apoprotein is assembled into a pigmentprotein complex which is electrophoretically indistinguishable from the native CPI complex. Isolated spinach chloroplasts also incorporate [³H]leucine and [ 35S]methionine into cytochrome b559. The radioactive label remains with the cytochrome through all stages of purification : extraction of the thylakoid membranes with Triton X-100 and urea, adsorption of impurities on DEAE cellulose, two cycles of electrophoresis in Triton-containing polyacrylamide gels and electrophoresis in SDS-containing gradient gels. Cytochrome b559 becomes labeled with both [3H]leucine and [35S]methionine and accounts for somewhat >1% of the total isotopic incorporation into thylakoid protein. The lipoprotein appears to be fully assembled during the time-course of our labeling experiments.