Interaction between the photoprotective protein LHCSR3 and C2S2 Photosystem II supercomplex in Chlamydomonas reinhardtii.

Photosynthetic organisms can thermally dissipate excess of absorbed energy in high-light conditions in a process known as non-photochemical quenching (NPQ). In the green alga Chlamydomonas reinhardtii this process depends on the presence of the light-harvesting protein LHCSR3, which is only expressed in high light. LHCSR3 has been shown to act as a quencher when associated with the Photosystem II supercomplex and to respond to pH changes, but the mechanism of quenching has not been elucidated yet. In this work we have studied the interaction between LHCSR3 and Photosystem II C2S2 supercomplexes by single particle electron microscopy. It was found that LHCSR3 predominantly binds at three different positions and that the CP26 subunit and the LHCII trimer of C2S2 supercomplexes are involved in binding, while we could not find evidences for a direct association of LHCSR3 with the PSII core. At all three locations LHCSR3 is present almost exclusively as a dimer.

Revisit of Reconstituted 30-nm Nucleosome Arrays Reveals an Ensemble of Dynamic Structures.

It has long been suggested that chromatin may form a fiber with a diameter of ~30 nm that suppresses transcription. Despite nearly four decades of study, the structural nature of the 30-nm chromatin fiber and conclusive evidence of its existence in vivo remain elusive. The key support for the existence of specific 30-nm chromatin fiber structures is based on the determination of the structures of reconstituted nucleosome arrays using X-ray crystallography and single-particle cryo-electron microscopy coupled with glutaraldehyde chemical cross-linking. Here we report the characterization of these nucleosome arrays in solution using analytical ultracentrifugation, NMR, and small-angle X-ray scattering. We found that the physical properties of these nucleosome arrays in solution are not consistent with formation of just a few discrete structures of nucleosome arrays. In addition, we obtained a crystal of the nucleosome in complex with the globular domain of linker histone H5 that shows a new form of nucleosome packing and suggests a plausible alternative compact conformation for nucleosome arrays. Taken together, our results challenge the key evidence for the existence of a limited number of structures of reconstituted nucleosome arrays in solution by revealing that the reconstituted nucleosome arrays are actually best described as an ensemble of various conformations with a zigzagged arrangement of nucleosomes. Our finding has implications for understanding the structure and function of chromatin in vivo.