Processing the Polyprotein Precursor to Euglena LHCPII

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The transport of cytoplasmically synthesized proteins to cellular organelles and insertion into membranes is a fundamental question confronting biologists. As a rule, chloroplast proteins are synthesized on free ribosomes and post-translationally imported into chloroplasts while Endoplasmic Reticulum (ER) proteins are synthesized on membrane bound ribosomes and co-translationally imported to the ER. Proteolytic precursor maturation and sorting for transport to their final intracellular site occurs in the ER and golgi. Immunoelectronmicroscopy has localized the Euglena chloroplast protein LHCPII to the golgi apparatus during times of active synthesis. Euglena pLHCPII is a polyprotein synthesized on membrane bound ribosomes. Photoinduction of Euglena LHCPII is translationally controlled at initiation or an early stage of elongation. Taken together, these results suggest that Euglena LHCPII is co-translationally transported into the ER and golgi apparatus prior to import into chloroplasts. This contrasts with the direct post-translational uptake of precursor proteins into higher plant and green algal chloroplasts. The purpose of this research is to use in vivo pulse labeling and organelle fractionation as well as in vivo translocation studies into microsmes and chloroplasts (independently and sequentially) to delineate the steps required and intracellular compartments involved in the novel pathway Euglena utilizes for the transport of cytoplasmically synthesized proteins to chloroplasts. %%% The assembly of chloroplast membranes requires the synthesis and transport of constituent proteins from various cellular compartments. This research will delineate the cellular site of synthesis of the photosynthetic enzyme complex "Light Harvesting Chlorophyll-Protein II" and elucidate the mechanism by which it is transported into the developing chloroplast structure. Information gained will increase our understanding of the biosynthesis and assembly of membrane-localized multienzyme complexes.