Photocontrol and processing of LHCP II apoprotein in Euglena: possible role of Golgi and other cytoplasmic sites.

ABSTRACT

Like other green photosynthetic eukaryotes, cells of Euglena gracilis var. bacillaris and strain Z contain a light-harvesting chlorophyll a/b complex associated with photosystem II. In Euglena, the formation of the 26.5 kDa principal light-harvesting chlorophyll a/b binding protein of photosystem II (LHCP II) has a number of unusual features. The precursors to LHCP II are large polyproteins containing multiple copies of LHCP II, and photocontrol of their formation is largely translational. Under conditions favoring LHCP II accumulation in the thylakoids, a reaction with anti-LHCP II antibody can be observed in the Golgi by immunogold electron microscopy. The timing of the immunoreaction in the Golgi in synchronous cells and in cells undergoing normal light-induced chloroplast development suggests that the nascent LHCP II passes through the Golgi on the way to the thylakoids. The compartmentalized osmiophilic structure (COS) also shows an immunoreaction. These observations, and other discussed in this paper, suggest that light permits translation of polyprotein LHCP II precursors on cytoplasmic ribosomes of the rough endoplasmic reticulum (ER) and that these pass through the ER to the Golgi where, presumably, further modifications take place. Since an LHCP II immunoreaction is found in Golgi vesicles, these may transport the nascent LHCP II to the plastid and facilitate its uptake.