Implications of a developmental-stage-dependent thylakoid-bound protease in the stabilization of the light-harvesting pigment-protein complex serving photosystem II during thylakoid biogenesis in red kidney bean

Biogenesis of light harvesting proteins

The LHC family includes nuclear-encoded, integral thylakoid membrane proteins, most of which coordinate chlorophyll and xanthophyll chromophores. By assembling with the core complexes of both photosystems, LHCs form a flexible peripheral moiety for enhancing light-harvesting cross-section, regulating its efficiency and providing protection against photo-oxidative stress. Upon its first appearance, LHC proteins underwent evolutionary diversification into a large protein family with a complex genetic redundancy. Such differentiation appears as a crucial event in the adaptation of photosynthetic organisms to changing environmental conditions and land colonization. The structure of photosystems, including nuclear- and chloroplast-encoded subunits, presented the cell with a number of challenges for the control of the light harvesting function. Indeed, LHC-encoding messages are translated in the cytosol, and pre-proteins imported into the chloroplast, processed to their mature size and targeted to the thylakoids where are assembled with chromophores. Thus, a tight coordination between nuclear and plastid gene expression, in response to environmental stimuli, is required to adjust LHC composition during photoacclimation. In recent years, remarkable progress has been achieved in elucidating structure, function and regulatory pathways involving LHCs; however, a number of molecular details still await elucidation. In this review, we will provide an overview on the current knowledge on LHC biogenesis, ranging from organization of pigment-protein complexes to the modulation of gene expression, import and targeting to the photosynthetic membranes, and regulation of LHC assembly and turnover. Genes controlling these events are potential candidate for biotechnological applications aimed at optimizing light use efficiency of photosynthetic organisms. This article is part of a Special Issue entitled: Chloroplast biogenesis.

High light stimulates Deg1-dependent cleavage of the minor LHCII antenna proteins CP26 and CP29 and the PsbS protein in Arabidopsis thaliana

The chloroplast Deg1 protein performs proteolytic cleavage of the photodamaged D1 protein of the photosystem II (PSII) reaction center, PSII extrinsic subunit PsbO and the soluble electron carrier plastocyanin. Using biochemical, immunological and mass spectrometry approaches we showed that the heterogeneously expressed Deg1 protease from Arabidopsis thaliana can be responsible for the degradation of the monomeric light-harvesting complex antenna subunits of PSII (LHCII), CP26 and CP29, as well as PSII-associated PsbS (CP22/NPQ4) protein. The results may indicate that cytochrome b (6) protein and two previously unknown thylakoid proteins, Ptac16 and an 18.3-kDa protein, may be the substrates for Deg1. The interaction of Deg1 with the PsbS protein and the minor LHCII subunits implies its involvement in the regulation of both excess energy dissipation and state transition adaptation processes.

Participation of chlorophyll b reductase in the initial step of the degradation of light-harvesting chlorophyll a/b-protein complexes in Arabidopsis

The light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) is the most abundant membrane protein in green plants, and its degradation is a crucial process for the acclimation to high light conditions and for the recovery of nitrogen (N) and carbon (C) during senescence. However, the molecular mechanism of LHCII degradation is largely unknown. Here, we report that chlorophyll b reductase, which catalyzes the first step of chlorophyll b degradation, plays a central role in LHCII degradation. When the genes for chlorophyll b reductases NOL and NYC1 were disrupted in Arabidopsis thaliana, chlorophyll b and LHCII were not degraded during senescence, whereas other pigment complexes completely disappeared. When purified trimeric LHCII was incubated with recombinant chlorophyll b reductase (NOL), expressed in Escherichia coli, the chlorophyll b in LHCII was converted to 7-hydroxymethyl chlorophyll a. Accompanying this conversion, chlorophylls were released from LHCII apoproteins until all the chlorophyll molecules in LHCII dissociated from the complexes. Chlorophyll-depleted LHCII apoproteins did not dissociate into monomeric forms but remained in the trimeric form. Based on these results, we propose the novel hypothesis that chlorophyll b reductase catalyzes the initial step of LHCII degradation, and that trimeric LHCII is a substrate of LHCII degradation.

Involvement of nitric oxide in light-mediated greening of barley seedlings

When seedlings are grown in the dark, proplastids of the developing leaf differentiate into etioplasts. Greening of etiolated plastids is stimulated by light, which is sensed by various types of photoreceptors. Nitric oxide (NO) has been shown to be a bioactive molecule that could take part in this light-mediated process in plants. In this paper, we show that emission of NO in barley seedlings increased concomitantly with increasing activities of nitric oxide synthase (NOS) during the greening. Treatment with sodium nitroprusside (SNP), a NO donor, increased the accumulation of chlorophyll contents, enhanced the accumulation of thylakoid membrane proteins, such as light harvesting complex of photosystem II (LHCII) and PSIA/B, and then improved the effective quantum yield of photosystem II (PSII) (Phi(PSII)) in the light. Instead, treatment with either NO scavenger 2-phenyl-4,4,5,5-tetramentylimidazoline-1-oxyl-3-xide (PTIO) or NOS inhibitor N(omega)-nitro-l-arginine (l-NNA) retarded the greening of etiolated-seedlings. Moreover, sodium ferrocyanide, an analog of SNP, nitrite and nitrate, two NO-decomposition products did not have any effect on the greening process. These results indicated that NO, as an endogenous signaling molecule, participates in light-mediated greening of barley seedlings, and exogenous NO accelerates this process.

Chlorophyll degradation during senescence

The catabolic pathway of chlorophyll (Chl) during senescence and fruit ripening leads to the accumulation of colorless breakdown products (NCCs). This review updates an earlier review on Chl breakdown published here in 1999 ( 69 ). It summarizes recent advances in the biochemical reactions of the pathway and describes the characterization of new NCCs and their formation inside the vacuole. Furthermore, I focus on the recent molecular identification of three chl catabolic enzymes, chlorophyllase, pheophorbide a oxygenase (PAO), and red Chl catabolite reductase (RCCR). The analysis of Chl catabolic mutants demonstrates the importance of Chl breakdown for plant development and survival. Mutants defective in PAO or RCCR develop a lesion mimic phenotype, due to the accumulation of breakdown intermediates. Thus, Chl breakdown is a prerequisite to detoxify the potentially phototoxic pigment within the vacuoles in order to permit the remobilization of nitrogen from Chl-binding proteins to proceed during senescence.

AtFtsH6 is involved in the degradation of the light-harvesting complex II during high-light acclimation and senescence

Degradation of the most abundant membrane protein on earth, the light-harvesting complex of Photosystem II (LHC II), is highly regulated under various environmental conditions, e.g., light stress, to prevent photochemical damage to the reaction center. We identified the LHC II degrading protease in Arabidopsis thaliana as a Zn(2+)-dependent metalloprotease, activated by the removal of unknown extrinsic factors, similar to the proteolytic activity directed against Lhcb3 in barley. By using a reversed genetic approach, the chloroplast-targeted protease FtsH6 was identified as being responsible for the degradation. T-DNA KO A. thaliana mutants, lacking ftsH6, were unable to degrade either Lhcb3 during dark-induced senescence or Lhcb1 and Lhcb3 during highlight acclimation. The A. thaliana ftsH6 gene has a clear orthologue in the genome of Populus trichocarpa. It is likely that FtsH6 is a general LHC II protease and that FtsH6-dependent LHC II proteolysis is a feature of all higher plants.

Degradation of the main Photosystem II light-harvesting complex

Many factors trigger the degradation of proteins, including changes in environmental conditions, genetic mutations, and limitations in the availability of cofactors. Despite the importance for viability, still very little is known about protein degradation and its regulation. The degradation of the most abundant membrane protein on Earth, the light-harvesting complex of Photosystem II (LHC II), is highly regulated under different environmental conditions, e.g. light stress, to prevent photochemical damage of the reaction center. However, despite major effort to identify the protease/proteases involved in the degradation of the apoproteins of LHC II the molecular details of this important process remain obscure. LHC II belongs to the family of chlorophyll a/b binding proteins (CAB proteins) and is located in the thylakoid membrane of the plant chloroplast. The results of biochemical experiments to isolate and characterize the protease degrading LHC II are summarized here and compared to our own recent finding indicating that a metalloprotease of the FtsH family is involved in this process.

Photoacclimation in spathiphyllum

We studied photoacclimation in Spathiphyllum grown at an irradiance of 40 or 420 micromol/m2 s (LL or HL, respectively). All parameters studied responded to acclimation. Leaves at LL, in contrast to HL, were thinner and oriented perpendicular to the incident light, had more chlorophyll per g f w, fewer stomata on the upper leaf surface and a reduced layer of mesophyll cells. Their chloroplasts at HL had wider grana with less thylakoids per granum, and better organized photosystems than at LL. PSI and PSII activities per mg chlorophyll ( Vmax ), and PSI and PSII content (total activity per g f w), were lower at LL than at HL and so was the light requirement for saturation of the PSI or PSII partial photoreactions, suggesting that fewer photosystems with larger antenna size prevail at LL, but many more with smaller antenna size at HL. Analysis of chlorophyll distribution among the thylakoid pigment-protein complexes showed less antenna chlorophyll serving PSII (CPa+LHCP1+LHCP3) than that serving PSI (CPIa+CPI+LHCP2) at LL as compared to HL, and thus a lower PSII/PSI ratio at LL, in agreement with the general finding that LL plants, with larger PSII antenna size, have lower PSII/PSI ratio. The increase in PSI antenna size at LL was correlated with the increase in the distribution of chlorophyll in pigment-protein complexes serving PSI, and a very large chlorophyll/protein molar ratio in the isolated CPI complex. On the other hand, the PSII antenna chlorophyll (CPa+LHCP1+LHCP3) on a g f w basis, and the chlorophyll a/b ratio remained more or less constant at LL or HL. This may reflect our finding that Spathiphyllum contains mainly the 27 kDa inner LHCII antenna protein, the size of which remains unaffected by photoacclimation. The increase in the distribution of chlorophyll in pigment-protein complexes serving PSII at HL, therefore, reflects the higher population of PSII at HL. Very high PSI activity was found at HL, which we attribute to the highly organized small in size PSI.

Senescence-dependent degradation of Lhcb3 is mediated by a thylakoid membrane-bound protease

A novel proteolytic activity integrally associated with barley thylakoid membranes has been discovered and characterized. This enzymatic activity mediates senescence-dependent degradation of Lhcb3, one of the apoproteins of the major light-harvesting chlorophyll a/b protein complex of photosystem II. Once senescence of barley leaves is initiated by detachment and dark incubation, the degradation of Lhcb3 can proceed and be followed in vitro in an experimental system composed of thylakoids isolated from senescing leaves incubated in darkness in suitable medium at 25 degrees C. The protease involved is present in its active form and Lhcb3 is susceptible for proteolytic attack already in fresh leaves, although Lhcb3 degradation does not take place unless undefined extrinsic membrane proteins protecting Lhcb3 are removed in a senescence-dependent manner. It is thus concluded that senescence-dependent Lhcb3 degradation is regulated at the substrate availability level. The protease involved is ATP stimulated, has an optimum activity at pH 7.8, and requires 3 mM added Mg2+ (replicable by micromolar doses of Zn2+) for its proper activity. Studies using typical inhibitors of various classes of proteases indicate that the enzyme is a metalloprotease with disulfide linkage essential for its activity. Micromolar doses of Zn2+ were demonstrated to restore the activity of Lhcb3-degrading enzymes abolished by an ethylenediaminetetraacetic acid pretreatment of the thylakoids and it is inferred that the protease involved is a zinc-binding metalloprotease. Mg2+ was shown to be able to partially replace zinc as the bound ion.

Formation of radicals from singlet oxygen produced during photoinhibition of isolated light-harvesting proteins of photosystem II

Electron spin resonance spectroscopy and liquid chromatography have been used to detect radical formation and fragmentation of polypeptides during photoinhibition of purified major antenna proteins, free of protease contaminants. In the absence of oxygen and light, no radicals were observed and there was no damage to the proteins. Similarly illumination of the apoproteins did not induce any polypeptide fragmentation, suggesting that chlorophyll, light and atmospheric oxygen are all participating in antenna degradation. The use of TEMP and DMPO as spin traps showed that protein damage initiates with generation of (1)O(2), presumably from a triplet chlorophyll, acting as a Type II photosensitizer which attacks directly the amino acids causing a complete degradation of protein into small fragments, without the contribution of proteases. Through the use of scavengers, it was shown that superoxide and H(2)O(2) were not involved initially in the reaction mechanism. A higher production of radicals was observed in trimers than in monomeric antenna, while radical production is strongly reduced when antennae were organized in the photosystem II (PSII) complex. Thus, monomerization of antennae as well as their incorporation into the PSII complex seem to represent physiologically protected forms. A comparison is made of the photoinhibition mechanisms of different photosynthetic systems.

Expression of xanthophyll biosynthetic genes during light-dependent chloroplast differentiation

In higher plants, etioplast to chloroplast differentiation is characterized by dramatic ultrastructural changes of the plastid and a concomitant increase in chlorophylls and carotenoids. Whereas the formation and function of carotenes and their oxygenated derivatives, the xanthophylls, have been well studied, little is known about the regulation of the genes involved in xanthophyll biosynthesis. Here, we analyze the expression of three xanthophyll biosynthetic genes (i.e. beta-carotene hydroxylase [bhy], zeaxanthin epoxidase [zep], and violaxanthin de-epoxidase [vde]) during de-etiolation of seedlings of tobacco (Nicotiana tabacum L. cv Samsun) under different light conditions. White-light illumination caused an increase in the amount of all corresponding mRNAs. The expression profiles of bhy and zep not only resembled each other but were also similar to the pattern of a gene encoding a major light-harvesting protein of photosystem II. This finding indicates a coordinated synthesis during formation of the antenna complex. In contrast, the expression pattern of vde was clearly different. Furthermore, the gene expression of bhy was shown to be modulated after illumination with different white-light intensities. The expression of all xanthophyll biosynthetic genes under examination was up-regulated upon exposure to red, blue, and white light. Gene expression of bhy and vde but not of zep was more pronounced under red-light illumination, pointing at an involvement of the phytochrome system. Expression analysis in the presence of the photosynthetic electron transport inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone indicated a redox control of transcription of two of the xanthophyll biosynthetic genes (bhy and zep).

Proteolytic activity against the light-harvesting complex and the D1/D2 core proteins of Photosystem II in close association to the light-harvesting complex II trimer

Light-harvesting complex II (LHCII) prepared from isolated thylakoids of either broken or intact chloroplasts by three independent methods, exhibits proteolytic activity against LHCII. This activity is readily detectable upon incubation of these preparations at 37 degrees C (without addition of any chemicals or prior pre-treatment), and can be monitored either by the LHCII immunostain reduction on Western blots or by the Coomassie blue stain reduction in substrate-containing "activity gels". Upon SDS-sucrose density gradient ultracentrifugation of SDS-solubilized thylakoids, a method which succeeds in the separation of the pigment-protein complexes in their trimeric and monomeric forms, the protease activity copurifies with the LHCII trimer, its monomer exhibiting no activity. This LHCII trimer, apart from being "self-digested", also degrades the Photosystem II (PSII) core proteins (D1, D2) when added to an isolated PSII core protein preparation containing the D1/D2 heterodimer. Under our experimental conditions, 50% of LHCII or the D1, D2 proteins are degraded by the LHCII-protease complex within 30 min at 37 degrees C and specific degradation products are observed. The protease is light-inducible during chloroplast biogenesis, stable in low concentrations of SDS, activated by Mg(2+), and inhibited by Zn(2+), Cd(2+), EDTA and p-hydroxy-mercury benzoate (pOHMB), suggesting that it may belong to the cysteine family of proteases. Upon electrophoresis of the LHCII trimer on substrate-containing "activity gels" or normal Laemmli gels, the protease is released from the complex and runs in the upper part of the gel, above the LHCII trimer. A polypeptide of 140 kDa that exhibits proteolytic activity against LHCII, D1 and D2 has been identified as the protease. We believe that this membrane-bound protease is closely associated to the LHCII complex in vivo, as an LHCII-protease complex, its function being the regulation of the PSII unit assembly and/or adaptation.

Identification and characterization of SppA, a novel light-inducible chloroplast protease complex associated with thylakoid membranes

A new component of the chloroplast proteolytic machinery from Arabidopsis thaliana was identified as a SppA-type protease. The sequence of the mature protein, deduced from a full-length cDNA, displays 22% identity to the serine-type protease IV (SppA) from Escherichia coli and 27% identity to Synechocystis SppA1 (sll1703) but lacks the putative transmembrane spanning segments predicted from the E. coli sequence. The N-terminal sequence exhibits typical features of a cleavable chloroplast stroma-targeting sequence. The chloroplast localization of SppA was confirmed by in organello import experiments using an in vitro expression system and by immunodetection with antigen-specific antisera. Subfractionation of intact chloroplasts demonstrated that SppA is associated exclusively with thylakoid membranes, predominantly stroma lamellae, and is a part of some high molecular mass complex of about 270 kDa that exhibits proteolytic activity. Treatments with chaotropic salts and proteases showed that SppA is largely exposed to the stroma but that it behaves as an intrinsic membrane protein that may have an unusual monotopic arrangement in the thylakoids. We demonstrate that SppA is a light-inducible protease and discuss its possible involvement in the light-dependent degradation of antenna and photosystem II complexes that both involve serine-type proteases.

Stable insertion of the early light-induced proteins into etioplast membranes requires chlorophyll a

Etiolated plant seedlings exposed to light respond by transient accumulation of the nucleus-encoded, plastid-located early light-inducible proteins (Elips). These proteins are distant relatives of the light-harvesting chlorophyll a/b-binding gene family and bind pigments with unusual characteristics. To investigate whether accumulation of Elips in plastid membranes is post-translationally regulated by pigments, reconstitution studies were performed, where in vitro transcribed and translated low molecular mass Elip precursors of barley were combined with lysed barley etioplasts complemented with various compositions of isolated pigments. We showed that the membrane insertion of Elips, as proven by protease protection assays and washes with a chaotropic salt or alkali, depended strictly on chlorophyll a but not on chlorophyll b or xanthophyll zeaxanthin. The amount of inserted Elips increased almost linearly with the chlorophyll a concentration, and the insertion efficiency was not significantly influenced by a light intensity between 1 and 1,000 micromol x m(-2) x s(-1). In contrast, in vitro import of Elip precursors into greening plastids was enhanced by high intensity light. Thus, we conclude that although chlorophylls bound to Elips seem to not be involved in light harvesting, they are crucial for a stable insertion of these proteins into the plastid membrane.

The N-terminal domain of the light-harvesting chlorophyll a/b-binding protein complex (LHCII) is essential for its acclimative proteolysis

Variations in the amount of the light-harvesting chlorophyll a/b-binding protein complex (LHCII) is essential for regulation of the uptake of light into photosystem II. An endogenous proteolytic system was found to be involved in the degradation of LHCII in response to elevated light intensities and the proteolysis was shown to be under tight regulation [Yang, D.-H. et al. (1998) Plant Physiol. 118, 827-834]. In this study, the substrate specificity and recognition site towards the protease were examined using reconstituted wild-type and mutant recombinant LHCII. The results show that the LHCII apoprotein and the monomeric form of the holoprotein are targeted for proteolysis while the trimeric form is not. The N-terminal domain of LHCII was found to be essential for recognition by the regulatory protease and the involvement of the N-end rule pathway is discussed.