Light-Harvesting Chlorophyll a/b Protein

Abstract

The apoprotein of the light-harvesting chlorophyll alb 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 IL 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.Thylakoid membranes contain approximately 45 different polypeptides, most of which are organized into four distinct supramolecular complexes: PSI, PSII, Cyt b/f, and the ATP synthase (11,23). In most plant chloroplasts, these complexes are nonuniformly distributed into two different structural thylakoid domains, the appressed membranes and the nonappressed membranes. PSII complexes are located predominantly in the appressed membranes, PSI and ATP synthase are located exclusively in the nonappressed membranes, and the Cyt blf complex appears to be present in both domains (23).Biogenesis of thylakoid protein complexes is an intriguing but little understood process. Each complex contains nuclearly encoded as well as chloroplast-encoded polypeptides (11). Thus, complex formation involves coordinate assembly of polypeptides that derive from two different cellular compartments. Chloroplast-encoded polypeptides are synthesized at or near their functional locations on thylakoid-bound ribosomes (21). Conversely, nuclearly encoded polypeptides must be imported from their site of synthesis in the cytosol (29). The fact that thylakoids are spatially separated from the delimiting envelope membranes indicates that assembly of these polypeptides is a multistep process, involving interaction with three different membranes. The assembly pathway for cytosolically synthesized thylakoid proteins has not been definitively established. However, recent studies (9, 30) suggest that these proteins are translocated across the envelope into the aqueous stromal matrix, where they are assembled into thylakoids by a second protein translocation/integration process.

Proteomics of Chlamydomonas reinhardtii Light-Harvesting Proteins

“…What could be the cause for the differential processing? Differential processing of pre-LHC proteins has already been reported in in vitro studies for wheat, pea, tobacco, tomato, and corn (5,6,8,27,35). A secondary processing site in the wheat sequence is believed to be within the mature protein (7) and is, interestingly, beyond Thr3, which has been found to be phosphorylated in thylakoid membranes from spinach and Arabidopsis (31,45).…”

Pigments induce folding of light‐harvesting chlorophyll a/b‐binding protein

“…This is not unexpected since LHCP and pLHCP can both be reconstituted with pigments in vitro to form complexes of very similar structure and stability (Paulsen and Hobe, 1992). Moreover, it has been shown that both LHCP and pLHCP can be assembled into LHCII in vitro (Chitnis et al, 1988;Cline 1988). In Fig.…”

A Single Precursor Protein for Ferrochelatase-I fromArabidopsis Is Imported in Vitro into Both Chloroplasts and Mitochondria

“…1C), which is found solely in the chloroplasts (15). Although the precursor associated with the mitochondria (lane 2), it was completely digested by Proteinase K (lane 3), suggesting that its association was through nonspecific interactions with the outer membrane.…”