In Vitro Processing of Precursors of Thylakoid Membrane Proteins of Chlamydomonas reinhardtii y-1

Localization of light-harvesting complex apoproteins in the chloroplast and cytoplasm during greening ofChlamydomonas reinhardtii at 38°C

Assembly of the major light-harvesting complex (LHC II) and development of photosynthetic function were examined during the initial phase of thylakoid biogenesis inChlamydomonas reinhardtii cells at 38°C. Continuous monitoring of LHC II fluorescence showed that these processes were initiated immediately upon exposure of cells to light. However, mature-size apoproteins of LHC II (Lhcb) increased in amount in an alkali-soluble (non-membrane) fraction in parallel with the increase in the membrane fraction. Alkali-soluble Lhcb were not integrated into membranes when protein synthesis was inhibited, suggesting that they were not active intermediates in LHC II assembly, nor were they recovered in a purified chloroplast preparation. Immunocytochemical analysis of greening cells revealed Lhcb inside the chloroplast near the envelope and in clusters deeper in the organelle. Antibody binding also detected Lhcb in granules within vacuoles in the cytosol, and Lhcb were recovered in granules purified from greening cells. Our results suggest that the cytosolic granules serve as receptacles of Lhcb synthesized in excess of the amount that can be accommodated by thylakoid membrane formation within the plastid envelope.

Biogenesis of thylakoid membranes with emphasis on the process in Chlamydomonas

Recent results obtained by electron microscopic and biochemical analyses of greening Chlamydomonas reinhardtii y1 suggest that localized expansion of the plastid envelope is involved in thylakoid biogenesis. Kinetic analyses of the assembly of light-harvesting complexes and development of photosynthetic function when degreened cells of the alga are exposed to light suggest that proteins integrate into membrane at the level of the envelope. Current information, therefore, supports the earlier conclussion that the chloroplast envelope is a major biogenic structure, from which thylakoid membranes emerge. Chloroplast development in Chlamydomonas provides unique opportunities to examine in detail the biogenesis of thylakoids.

Purification and Characterization of a Membrane-Bound Protease from Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii y-1, newly synthesized chlorophyll a/b-binding apoproteins are degraded when chlorophylls are not present for assembly of stable light-harvesting complexes. A protease was purified from the membrane fraction of degreened y-1 cells, which digested chlorophyll a/b-binding proteins in membranes from C. reinhardtii pg-113, a protease-deficient strain. This protease was active with p-nitroanilides of nonpolar amino acids (Leu and Phe), but not of basic amino acids (Lys and Arg). The apparent molecular weight of the enzyme is 38,000 +/- 2,000 as determined by electrophoresis in the presence of sodium dodecyl sulfate. Typical inhibitors of the major classes of proteases were ineffective with this enzyme. Protease activity was constant from pH 7.5 to 9; a plot of log V versus pH suggested that deprotonation of an ionizable group with a pK value of 6.0 to 6.5 is required for activity. The protease was inactivated by diethylpyrocarbonate and by photooxidation sensitized by rose bengal. These results suggested that a histidyl residue is required for catalysis. Although very sensitive to photodynamic conditions in vitro, the enzyme was not inactivated in vivo when cells were exposed to light.

Accumulation of Chlorophyll a/b-Binding Polypeptides in Chlamydomonas reinhardtii y-1 in the Light or Dark at 38 degrees C : Evidence for Proteolytic Control

The kinetics of accumulation of light harvesting chlorophyll (Chl) a/b-binding polypeptides (LHCPs) in thylakoid membranes were analyzed during greening of Chlamydomonas reinhardtii y-1 at 38 degrees C. Initial accumulation of LHCPs in thylakoid membranes was linear; LHCP precursors or polypeptides in transit within the chloroplast stroma were not detected. The rate of accumulation in the light was at least five-fold greater than that in the dark. The relatively small amount of LHCPs that accumulated in the dark was integrated properly in the membrane, as judged by the pattern of cleavage in vitro by exogenous proteases, and did not turn over at a significant rate in vivo. The kinetic data suggested that in y-1 cells either translation of LHCP mRNA was inhibited in the dark or newly synthesized polypeptides were degraded concurrently with transport into the chloroplast unless rescued by Chl. LHCPs accumulated in cells of the Chl b-deficient strain pg-113 at the same rate in the dark or the light at 38 degrees C, an indication that light did not affect translation of LHCP mRNA. Membrane-associated LHCPs in pg-113 cells were completely degraded, in contrast to those in y-1 cells, by exogenous proteases, which suggested that pg-113 cells are deficient in a proteolytic activity. A peptidase was recovered from y-1 cells in a membrane fraction with a buoyant density slightly less than that of thylakoid membranes. Although a role for this activity in degradation of LHCPs has not been established, the specific activity of this peptidase in pg-113 cells was only 10 to 15% of the level in y-1 cells.

Light-Harvesting Chlorophyll a/b Protein : Membrane Insertion, Proteolytic Processing, Assembly into LHC II, and Localization to Appressed Membranes Occurs in Chloroplast Lysates

The apoprotein of the light-harvesting chlorophyll a/b protein (LHCP) is a major integral thylakoid membrane protein that is normally complexed with chlorophyll and xanthophylls and serves as the antenna complex of photosystem II. LHCP is encoded in the nucleus and synthesized in the cytosol as a higher molecular weight precursor that is subsequently imported into chloroplasts and assembled into thylakoids. In a previous study it was established that the LHCP precursor can integrate into isolated thylakoid membranes. The present study demonstrates that under conditions designed to preserve thylakoid structure, the inserted LHCP precursor is processed to mature size, assembled into the LHC II chlorophyll-protein complex, and localized to the appressed thylakoid membranes. Under these conditions, light can partially replace exogenous ATP in the membrane integration process.

Recent developments in chloroplast protein transport

Most proteins located in chloroplasts are encoded by nuclear genes, synthesized in the cytoplasm, and transported into the organelle. The study of protein uptake by chloroplasts has greatly expanded over the past few years. The increased activity in this field is due, in part, to the application of recombinant DNA methodology to the analysis of protein translocation. Added interest has also been gained by the realization that the transport mechanisms that mediate protein uptake by chloroplasts, mitochondria and the endoplasmic reticulum display certain characteristics in common. These include amino terminal sequences that target proteins to particular organelles, a transport process that is mechanistically independent from the events of translation, and an ATP-requiring transport step that is thought to involve partial unfolding of the protein to be translocated. In this review we examine recent studies on the binding of precursors to the chloroplast surface, the energy-dependent uptake of proteins into the stroma, and the targeting of proteins to the thylakoid lumen. These aspects of protein transport into chloroplasts are discussed in the context of recent studies on protein uptake by mitochondria.

Processing of a wheat light-harvesting chlorophyll a/b protein precursor by a soluble enzyme from higher plant chloroplasts

A processing activity has been identified in higher plant chloroplasts that cleaves the precursor of the light-harvesting chlorophyll a/b-binding protein (LHCP). A wheat LHCP gene previously characterized (Lamppa, G.K., G. Morelli, and N.-H. Chua, 1985. Mol. Cell Biol. 5:1370-1378) was used to synthesize RNA and subsequently the labeled precursor polypeptide in vitro. Incubation of the LHCP precursors with a soluble extract from lysed chloroplasts, after removal of the thylakoids and membrane vesicles, resulted in the release of a single 25-kD peptide. In contrast, when the LHCP precursors were used in an import reaction with intact pea or wheat chloroplasts, two forms (25 and 26 kD) of mature LHCP were found. The peptide released by the processing activity in the organelle-free assay comigrated with the lower molecular mass form of mature LHCP produced during import. Properties of the processing activity suggest that it is an endopeptidase. Chloroplasts from both pea and wheat, two divergent higher plants, contain the processing enzyme, suggesting its physiological importance in LHCP assembly into the thylakoids. We discuss the implications of LHCP precursor processing by a soluble enzyme that may be in the stromal compartment.

A Secondary Processing Site in the Precursor of the Small Subunit of Ribulose Bisphosphate Carboxylase of Chlamydomonas reinhardtii y-1

When the precursor of ribulose bisphosphate carboxylase of Chlamydomonas reinhardtii y-1 is bound to antibodies and treated with the soluble cell fraction, it is cleaved to the mature form (M(r) 16,500) via an intermediate of M(r) 18,500. Although this intermediate has only been observed in vitro, it may be produced during processing of the precursor in vivo.