The apparatus of photosynthetic energy conversion in chloroplasts is quite well characterized with respect to structure and function. Light-driven electron transport in the thylakoid membrane is coupled to synthesis of ATP, used to drive energy-dependent metabolic processes in the stroma and the outer surface of the thylakoid membrane. The role of the inner (luminal) compartment of the thylakoids has, however, remained largely unknown although recent proteomic analyses have revealed the presence of up to 80 different proteins. Further, there are no reports concerning the presence of nucleotides in the thylakoid lumen. Here, we bring three lines of experimental evidence for nucleotide-dependent processes in this chloroplast compartment. T he thylakoid membrane of chloroplasts is the site for the photosynthetic electron transport coupled to ATP synthesis (1). This membrane is surrounded by the soluble stroma, which contains the enzymes involved in CO 2 fixation, and it encloses the luminal space. In contrast to the other chloroplast compartments, the thylakoid lumen has been considered to have a limited functional significance for the photosynthetic process and mainly viewed as a sink for protons from a chemio-osmotic perspective. Until recently the protein composition of the thylakoid lumen was thought to be very simple and dominated by three extrinsic photosystem (PS) II proteins and plastocyanin (2). In the last few years, however, many different categories of proteins have been found in the thylakoid lumen by applying biochemical and proteomic approaches, pointing to several unexpected functions for this chloroplast compartment. Thus, the thylakoid lumen has been found to contain chaperones (3), immunophilins (4), carbonic anhydrases (5), violaxanthin deepoxidases (6), peroxidases (7), and proteases (8). Furthermore, systematic mass spectrometric analyses after two-dimensional electrophoretic separation of luminal preparations combined with prediction of transit peptides estimated the existence of Ϸ80 different thylakoid luminal proteins of Arabidopsis thaliana (9, 10), out of which only half have been assigned a putative function.This considerably more complex view of the thylakoid lumen raises several questions of mechanistic and physiological nature related to chloroplast function and regulation. One of these questions concerns the presence of nucleotides in such a potentially multifunctional cellular compartment as particularly suggested by indications for luminal chaperones (3, 9) and ATP binding to luminal proteins (11). On the other hand, luminal preparations have been tested for presence of ATP and ATPase activity without any conclusive results (12), and none of the proteins identified through proteomics have shown to have any conventional nucleotide-binding motifs (10). Furthermore, transport of nucleotides across the thylakoid membrane has not been considered, in contrast to mitochondrial membranes, where an ATP͞ADP carrier (AAC) has been extensively studied (13,14).Chloroplast metabolism has mainly ...

The superfamily of light-harvesting chlorophyll a/b-binding (Lhc) proteins in higher plants and green algae is composed of more than 20 different antenna proteins associated either with photosystem I (PSI) or photosystem II (PSII). Several distant relatives of this family with conserved chlorophyll-binding residues and proposed photoprotective functions are induced transiently under various stress conditions. Whereas "classical" Lhc proteins contain three-transmembrane ␣-helices, their distant relatives span the membrane with between one and four transmembrane segments. Here, we report the identification and isolation of a novel member of the Lhc family from Arabidopsis with one predicted transmembrane ␣-helix closely related to helix I of Lhc protein from PSI (Lhca4) that we named Ohp2 (for a second one-helix protein of Lhc family described from higher plants). We showed that the Ohp2 gene expression is triggered by light stress and that the Ohp2 transcript and protein accumulated in a light intensity-dependent manner. Other stress conditions did not up-regulate the expression of the Ohp2 gene. Localization studies revealed that Ohp2 is associated with PSI under low-or high-light conditions. Because all stress-induced Lhc relatives reported so far were found in PSII, we propose that the accumulation of Ohp2 might represent a novel photoprotective strategy induced within PSI in response to light stress.The superfamily of light-harvesting chlorophyll a/b-binding (Lhc) proteins in higher plants and green algae is composed of more than 20 different members associated with photosystem I (PSI) or photosystem II (PSII). The primary function of these proteins is the absorption of light through chlorophyll excitation and the transfer of the absorbed energy to photochemical reaction centers (Green and Durnford, 1996). On the basis of the three-dimensional structure determined at 3.4-Å resolution for one member of the Lhc family from higher plants (Kü hlbrandt et al., 1994), it was proposed that all Lhc proteins in higher plants and green algae have three transmembrane ␣-helices, where helices I and III are evolutionarily related to each other and held together by ion pairs formed by charged residues.In the past few years, several distant relatives of Lhc protein family with conserved chlorophyllbinding residues and a transient expression pattern related to various stress conditions have been described from higher plants, algae, or cyanobacteria (Adamska, 2001). These distant relatives include the four-helix PsbS protein of PSII (Funk, 2001) and a subfamily of proteins called early light-induced proteins (Elips; Adamska, 1997Adamska, , 2001 Montané and Kloppstech, 2000). The Elip subfamily consists of three-helix Elips, two-helix stress-enhanced proteins (Seps), and one-helix proteins (Ohps) also called high-light-induced proteins (Hlips) or small chlorophyll a/b-binding-like proteins (Scps) in prokaryotic organisms (Adamska, 2001). All Elip subfamily members are short-lived proteins (Meyer and Kloppstech, 1984; Grim...

The early light-induced proteins (Elips) in higher plants are nuclear-encoded, light stress-induced proteins located in thylakoid membranes and related to light-harvesting chlorophyll (LHC) a/b-binding proteins. A photoprotective function was proposed for Elips. Here we showed that after 2 h exposure of Arabidopsis (Arabidopsis thaliana) leaves to light stress Elip1 and Elip2 coisolate equally with monomeric (mLhcb) and trimeric (tLhcb) populations of the major LHC from photosystem II (PSII) as based on the Elip:Lhcb protein ratio. A longer exposure to light stress resulted in increased amounts of Elips in tLhcb as compared to mLhcb, due to a reduction of tLhcb amounts. We demonstrated further that the expression of Elip1 and Elip2 transcripts was differentially regulated in green leaves exposed to light stress. The accumulation of Elip1 transcripts and proteins increased almost linearly with increasing light intensities and correlated with the degree of photoinactivation and photodamage of PSII reaction centers. A stepwise accumulation of Elip2 was induced when 40% of PSII reaction centers became photodamaged. The differential expression of Elip1 and Elip2 occurred also in light stress-preadapted or senescent leaves exposed to light stress but there was a lack of correlation between transcript and protein accumulation. Also in this system the accumulation of Elip1 but not Elip2 correlated with the degree of PSII photodamage. Based on pigment analysis, measurements of PSII activity, and assays of the oxidation status of proteins we propose that the discrepancy between amounts of Elip transcripts and proteins in light stress-preadapted or senescent leaves is related to a presence of photoprotective anthocyanins or to lower chlorophyll availability, respectively.