Transfer of proteins from the chloroplast to vacuoles in Chlamydomonas reinhardtii (Chlorophyta): A pathway for degradation

Several chloroplast proteins were detected by immunoelectron microscopy wit hin dense granules in cytoplasmic vacuoles in the alga Chlamydomonas reinha rdtii Dangeard. Transfer from chloroplast to vacuoles of two major, pulse-l abeled polypeptides, the large subunit of rubisco and the or subunit of ATP ase, which are synthesized on chloroplast ribosomes, was demonstrated by th e recovery of these polypeptides in vacuolar granules over a several-hour t ime period. The ultrastructure of cryofixed algal cells was examined to sea rch for structures that would provide insight into the transfer of chloropl ast proteins to vacuoles. Micrographs showed that the two membranes of the envelope were appressed, with no detectable intermembrane space, over most of the chloroplast surface. Protrusions of the outer membrane of the envelo pe were occasionally found that enclosed stroma, with particles similar in size to chloroplast ribosomes, but generally not thylakoid membranes. These observations suggest that Chloroplast material, especially the stromal pha se, was extruded from the chloroplast in membrane-bound structures, which t hen interacted with Golgi-derived vesicles for degradation of the contents by typical lysosomal activities. A protein normally targeted to vacuoles th rough the endomembrane system for incorporation into the cell wall was dete cted in Golgi structures and vacuolar granules but not the chloroplast.