Evidence for a selective destabilization of an integral membrane protein, the cytochromeb6/fcomplex, during gametogenesis inChlamydomonas reinhardtii

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Abstract
We studied the process of photosynthetic inactivation during gametogenesis of the unicellular green algaChlamydomonas reinhardtii. We show that it is caused by the selective destabilization of a single transmembrane protein complex, the cytochromeb6/fcomplex, which is initially accumulated in the thylakoid membranes of vegetative cells. This protein destabilization is controlled by the intracellular energy sources available in the gametes, i. e. the coupled electron flow in the mitochondria and the amount of starch accumulated in the chloroplast. It nevertheless requires the expression of gamete‐specific proteins. The loss of cytochromeb6/fcomplexes during gametogenesis is prevented by the addition of cycloheximide, but is chloramphenicol insensitive. Therefore, it is likely to involve some translation product of nuclear origin, specifically expressed during gametogenesis. Among the new polypeptides specifically found in the gametes, we detected a soluble polypeptide Mα (approximate molecular mass of 63 kDa), which shared common epitopes with cytochromef.Its synthesis displays an antibiotic sensitivity typical of a nuclear‐encoded polypeptide and is controlled by the same intracellular signals which control the destabilization of the cytochromeb6/fcomplexes in the thylakoid membranes.