Higher Plant Light‐Harvesting Complexes LHCIIa and LHCIIc are Bound by Dicyclohexylcarbodiimide During Inhibition of Energy Dissipation

Abstract

We have investigated the binding to proteins of the photosynthetic apparatus of the carboxymodifying agent dicyclohexylcarbodiimide, (cHxN),C ; this inhibits the protective dissipation of excess absorbed light energy (qE) by the light-harvesting apparatus of photosystem I1 (LHCII), suggesting that carboxyl amino-acid side chains within hydrophobic protein domains may be involved in qE. (cHxN),I4C was used to label thylakoids and photosystem I1 particles, so as to identify proteins which may be involved in the detection of lumen pH during qE induction. Of six thylakoid proteins labelled with (cHxN),C under conditions where qE is efficiently induced, three are associated with photosystem I, and none with the bulk LHCII. PSII-associated label is bound to three minor components of LHCII, identified as LHCIIa (two species) and LHCIIc, as shown by protein sequencing of tryptic fragments of purified complexes. pH titration of qE formation and protein labelling in coupled thylakoids showed that both qE and labelling of LHCIIa increased at pH 7-8.In higher plants, chlorophyll and carotenoid molecules are bound to specific proteins to form light-harvesting complexes (LHCI, LHCII), which efficiently capturc and deliver excitation energy to photosystem I (PSI) and photosystem I1 (PSII) reaction centres. Four different types of LHCII (LHCIIa-d), which differ in their polypeptide and pigment content, have been identified (Peter and Thornber, 1991). The polypeptides comprising LHCII are encoded by members of the Lhc gene family; Lhcbl, Lhcb2 and Lhcb3 encode 28-, 27-and 25-kDa polypeptides, respectively, comprising LHCIIb; Lhcb4 encodes the 30-31-kDa protein of LHCIIa; Lhcb.5 encodes the LHCIIc 26-kDa protein; the LHCIId 24-kDa protein is encoded by Lhcbd . LHCIIb binds up to 65% of PSII chlorophyll and is often referred to as the bulk light-harvesting complex; LHCIIa, c and d are minor complexes, each accounting for 3-5% (Peter and Thornber, 1991 ;Dainese and Bassi, 1991 ;.When the light absorbed by LHCII is greater than that capable of being used in photosynthesis, the photosynthetic apparatus is protected from the damaging effects of overexcitation by the rapid induction of a process of non-radiative energy dissipation known as qE Demmig-Adams, 1990;Horton and Ruban, 1992). Measurement of radiationless dissipation of energy via non-photochemical

Antisense Inhibition of the Photosynthetic Antenna Proteins CP29 and CP26: Implications for the Mechanism of Protective Energy Dissipation

“…CP29 antisense plants showed only marginal changes in the PSII antenna other than the loss of CP29 and a fraction of CP24. The loss of CP26 or CP29 had no effect on the acclimation of the PSII antenna to light quantity; for all lines, growth light-dependent changes in LHCII content were equivalent to between 1 and 1.5 extra trimers per PSII in LL-grown plants, consistent with previous observations for Arabidopsis and other species (Mäenpää and Andersson, 1989;Walters et al, 1994;.…”

The Xanthophyll Cycle Modulates the Kinetics of Nonphotochemical Energy Dissipation in Isolated Light-Harvesting Complexes, Intact Chloroplasts, and Leaves of Spinach1

“…Demmig-Adams and Adams, 1996;Gilmore et al, 1996b), and some have suggested that the minor LHCII components play a major role in qE in vivo Bassi et al, 1993;Crofts and Yerkes, 1994;Walters et al, 1994;Gilmore et al, 1996b;Pesaresi et al, 1997). The doubt about the involvement of LHCIIb is based partly on the study of mutants and/or developing chloro- plasts, which lack LHCIIb but still show qE (Härtel and Lokstein, 1995;Jahns and Schweig, 1995;Gilmore et al, 1996b).…”

“…Various reports suggest that qE occurs in LHCII Horton et al, 1996), where the xanthophyll cycle carotenoids are bound (Peter and Thornber, 1991; Bassi et al, 1993;Ruban et al, 1994b) and where proton-active sites involved in qE have been identified Pesaresi et al, 1997). It has been suggested that CP26 and CP29, two of the minor components of LHCII, have a major role in qE Bassi et al, 1993; Crofts and Yerkes, 1994;Walters et al, 1994;Pesaresi et al, 1997;Gilmore et al, 1996b).…”

“…Various reports suggest that qE occurs in LHCII Horton et al, 1996), where the xanthophyll cycle carotenoids are bound (Peter and Thornber, 1991; Bassi et al, 1993;Ruban et al, 1994b) and where proton-active sites involved in qE have been identified Pesaresi et al, 1997). It has been suggested that CP26 and CP29, two of the minor components of LHCII, have a major role in qE Bassi et al, 1993; Crofts and Yerkes, 1994;Walters et al, 1994;Pesaresi et al, 1997;Gilmore et al, 1996b).It is unclear how the xanthophyll cycle and qE are related. Some have suggested that the de-epoxidized pigments antheraxanthin and zeaxanthin directly quench excited chlorophyll singlet states (Demmig-Adams, 1990;Owens et al, 1992;Frank et al, 1994;Gilmore et al, 1995Gilmore et al, , 1996aGilmore et al, , 1996b.…”

Orientation of Chlorophyll Transition Moments in the Higher-Plant Light-Harvesting Complex CP29

“…The successful reconstitution of the minor Chl a and b antenna complexes, CP29, CP26, and CP24 using maize Lhcb4, -5, and -6 apoproteins overexpressed in Escherichia coli, was recently reported (14)(15)(16). The recombinant proteins exhibit absorption, fluorescence emission, and CD signals which closely match those of the native complex while even peculiar biochemical features such as DCCD binding (17) were reproduced (18). The recombinant CP29 system can therefore be used for the experimental analysis of the relevant parameters that have been left unknown after the structural work on LHCII by Kuhlbrandt and co-workers.…”