Phosphorylation Controls the Three-dimensional Structure of Plant Light Harvesting Complex II

Abstract

The most abundant chlorophyll-binding complex in plants is the intrinsic membrane protein light-harvesting complex II (LHC II). LHC II acts as a light-harvesting antenna and has an important role in the distribution of absorbed energy between the two photosystems of photosynthesis. We used spectroscopic techniques to study a synthetic peptide with identical sequence to the LHC IIb N terminus found in pea, with and without the phosphorylated Thr at the 5th amino acid residue, and to study both forms of the native full-length protein. Our results show that the N terminus of LHC II changes structure upon phosphorylation and that the structural change resembles that of rabbit glycogen phosphorylase, one of the few phosphoproteins where both phosphorylated and non-phosphorylated structures have been solved. Our results indicate that phosphorylation of membrane proteins may regulate their function through structural protein-protein interactions in surface-exposed domains. Light harvesting complex II (LHC II)1 is a major chlorophyllcontaining protein complex that accounts alone for half of the pigments involved in photosynthesis in plants. It is located mainly in appressed regions of the thylakoid membrane where it acts as the light-harvesting antenna for photosystem II (PS II). Reversible phosphorylation of LHC II is an established mechanism for redistribution of absorbed light energy between PS II and PS I. Phosphorylation of LHC II (giving LHC II(P)) is triggered by conditions where the plastoquinone pool of the photosynthetic electron transport chain becomes reduced (1). The kinase responsible for the phosphorylation of LHC II is not yet identified, although it is suggested that it is located in the core of photosystem II (2) or in contact with the cytochrome b 6 f complex (3, 4). LHC II(P) is found in the unappressed regions of the chloroplast thylakoid membrane and there acts as a lightharvesting antenna for photosystem I (PS I) (5-7). From electron crystallography of 2-dimensional crystals, a structure for the major part of non-phosphorylated LHC II has been described at 3.4-Å resolution (8). This structure reveals no information regarding the N-terminal domain that contains the phosphorylation site at position 5 (Thr); the protein backbone was traced only to residue 26 where it ends up close to the lipid membrane, consistent with the fact that the sequence between residues 21 and 29 (RVKYLGPF) (9) consists mainly of hydrophobic, aromatic, or charged amino acids. Aromatic residues are located at the membrane surface in structures of membrane proteins (10 -13), and residues Trp-16 and Tyr-17 of LHC II may also then form a point of contact with the membrane. LHC II has been shown to lose its ability to trimerize when more than the first 15 amino acids are removed from the apoprotein (14). At this site, specific lipid-protein interactions between the amino side chains and the lipid phosphatidylglycerol are involved in stabilization of the trimers (15), which implies that only the first 15 amino acid residues at the...

The Chloroplast Grana Proteome Defined by Intact Mass Measurements from Liquid Chromatography Mass Spectrometry

“…However, yield of these peptides can be influenced by trypsin accessibility, and thylakoid protein conformation is known to be affected by phosphorylation (39,40); furthermore, there is uncertainty about the efficiency of recovery of low abundance phosphopeptides by the immobilized metal ion chromatography procedure. 6 Therefore, care must be exercised in the interpretation of results obtained by this experimental method.…”

Molecular Remodeling of Photosystem II during State Transitions inChlamydomonas reinhardtii

“…As has been previously described by Dekker and Boekema (2005), the two C 2 S 2 supercomplexes constituting the megacomplex are bridged by an LHCII trimer in C. reinhardtii. Therefore, the phosphorylation of the most abundant trimeric LHCII, type I, may induce the division of the megacomplex due to its altered conformation (Sprang et al, 1988;Nilsson et al, 1997).…”

“…Notably, the roles of phosphorylated LHCII have been obscure. It was originally proposed that the phosphorylation of LHCII induces conformational changes, leading to its dissociation from PSII (Nilsson et al, 1997), but an Arabidopsis mutant lacking the H subunit of PSI shows that phosphorylated LHCIIs remain attached to PSII (Lunde et al, 2000). It was thus proposed that phosphorylation of LHCII increases its affinity for PSI over PSII and results in its migration from PSII to PSI (Allen and Forsberg, 2001).…”

ArabidopsisSTN7 Kinase Provides a Link between Short- and Long-Term Photosynthetic Acclimation

“…State transitions occur in the order of minutes and have been proposed to be triggered by conformational changes within LHCII proteins upon phosphorylation (Nilsson et al, 1997), leading to docking of phosphorylated LHCII (P i -LHCII) to PSI. The docking most likely involves the H, L, and O subunits of PSI, as demonstrated by the impairment of state transitions in the corresponding Arabidopsis thaliana null mutants (Lunde et al, 2000;Jensen et al, 2004).…”