Accumulation of a light-harvesting chlorophyll a/b protein in the chloroplast grana lamellae. The lateral migration of the membrane protein precursor is independent of its processing

The events that follow the import of pLHCPIIb, the apoprotein precursor of the major light-harvesting complex of photosystem II, were studied in intact pea chloroplasts. The distribution of the events of insertion into the membrane, and processing, to yield the mature form (LHCP) between stromal and granal lamellae regions of the thylakoids were followed. pLHCP was preferentially inserted into stromal lamellae (SL) from which it migrated to granal lamellae (GL). Migration occurred before or after processing, suggesting that migration and processing are independent of each other. When migration was slowed down, LHCP accumulated in SL. Prolonged inhibition of migration induced degradation of LHCP that had accumulated in SL, whereas inhibition of processing did not affect the migration of pLHCP into GL. A small difference in electrophoretic mobility was noted between LHCP in SL and in GL. The predominant mature form in SL migrated more slowly than LHCP from GL. When thylakoids were subjected to trypsin, all of the LHCP embedded in SL underwent cleavage, whereas up to 60% of the radioactive LHCP in GL was resistant to the enzyme. The possible implications of the differences in size and in the sensitivity to trypsin of LHCP are discussed