Abstract
The conformational distribution of the N-terminal domain of the major light-harvesting chlorophyll a/b protein (LHCIIb) has been characterized by electron-electron double resonance yielding distances between spin labels placed in various domains of the protein. Distance distributions involving residue 3 near the N terminus turned out to be bimodal, revealing that this domain, which is involved in regulatory functions such as balancing the energy flow through photosystems (PS) I and II, exists in at least two conformational states. Models of the conformational sub-ensembles were generated on the basis of experimental distance restraints from measurements on LHCIIb monomers and then checked for consistency with the experimental distance distribution between residues 3 in trimers. Only models where residue 3 is located above the core of the protein and extends into the aqueous phase on the stromal side fit the trimer data. In the other state, which consequently is populated only in monomers, the N-terminal domain extends sideways from the protein core. The two conformational states may correspond to two functional states of LHCIIb, namely trimeric LHCIIb associated with PSII in stacked thylakoid membranes and presumably monomeric LHCIIb associated with PSI in nonstacked thylakoids. The switch between these two is known to be triggered by phosphorylation of Thr-6. A similar phosphorylation-induced conformational change of the N-terminal domain has been observed by others in bovine annexin IV which, due to the conformational switch, also loses its membrane-aggregating property.
Highlights
The major light-harvesting chlorophyll a/b complex (LHCIIb)1 in higher plants contributes to photosynthesis most significantly by increasing the amount of light energy absorbed and of the energy flow through the photosynthetic reaction centers
As well as four carotenoids per apoprotein, which are noncovalently bound to the protein and positioned such that the excitation energy can rapidly be conducted toward the reaction centers
Interpretation of Distance Distributions and Structural Modeling—Pulse-EPR has been used as a “spectroscopic ruler” to measure intra- and intermolecular distances in monomeric and trimeric LHCIIb, respectively
Summary
The major light-harvesting chlorophyll a/b complex (LHCIIb) in higher plants contributes to photosynthesis most significantly by increasing the amount of light energy absorbed and of the energy flow through the photosynthetic reaction centers. One of the carotenoid components of the Chl a/b antenna of PSII, violaxanthin, is converted to zeaxanthin by a de-epoxidase residing in the lumen, and this conversion, together with the acidification of the lumen compartment, enables the light-harvesting apparatus to dissipate excitation energy into heat rather than funneling it to the reaction centers This quenching process requires the PSII protein PsbS [6] and is most likely brought about by a Chl-zeaxanthin heterodimer undergoing charge separation [7]. Spin labels used for EPR, such as tetramethylpiperidine-1-oxyl (TEMPO), compare in size to amino acid side chains and are less likely to interfere with the structural behavior of the protein Distances between such labels up to 20 Å can be measured with good precision by conventional continuous-wave EPR techniques [13]. By applying double-quantum EPR to the soluble protein lysozyme T4 as a model system, Borbat et al [23] demonstrated that EPR distance measurements combined with triangulation techniques can potentially be used to determine structural features of a protein
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