The Photosystem II Subunit S Dynamics under stress
Date Issued
September 2017
Author(s)
Abstract
The increased spectral range absorption of light exerted by pigments within Light
Harvesting Complexes (LHCs) proves an important advantage under low light conditions,
in higher plants. However, in the exposure to excess light, oxidative damages and
ultimately cell death can occur. It proved, thus, utmost important for the
photosynthetic organisms to develop a down-regulatory mechanism called NonPhotochemical
Quenching (NPQ). Quantifying this mechanism at the atomic level is
still very uncertain. There are several components of the photosynthetic apparatus
that are actively involved in NPQ. Apart from the LHCs, and the xanthophyll cycle,
the Photosystem II Subunit S (PsbS) is a 22-kDa integral membrane protein that is
essential for the response of the photosynthetic apparatus to high-light and it is
activated by the protonation of key lumen-exposed glutamate residues. Atomistic
details on its involvement in NPQ remain still a mystery. However, It is widely
accepted that NPQ (qE) is co-regulated by low lumen pH and ion fluxes (K+
, Ca2+, Mg2+
, Cl)
in Lumen-Stroma areas. It has also been proposed that the activated PsbS may
strongly interact with some LHCs enabling quenching by providing an alternative
environment for some pigments within these LHCs, or by changing the membrane
organization and dynamics. In this study, PsbS (pdb code 4ri2) is embedded in a lipid
bilayer model membrane (400-500 POPC lipids, 1-palmitoyl-2-oleoyl-sn-glycero-3-
phosphocholine). NPQ conditions are simulated by perturbations in the thylakoid lumen
ionic load. Zeaxanthin (Zea) of the xanthophyll cycle that is produced under NPQ is
also embedded in the membrane. We employ large-scale Molecular Dynamics simulations
to probe the PsbS conformational changes, membrane dynamics, or Zea binding that
activate PsbS. We identify two distinct PsbS forms (active-inactive), in response to
A) the lumen acidification or ion fluxes, and B) the Zea binding, revealing a PsbSNPQ
relation at the atomic scale.
Harvesting Complexes (LHCs) proves an important advantage under low light conditions,
in higher plants. However, in the exposure to excess light, oxidative damages and
ultimately cell death can occur. It proved, thus, utmost important for the
photosynthetic organisms to develop a down-regulatory mechanism called NonPhotochemical
Quenching (NPQ). Quantifying this mechanism at the atomic level is
still very uncertain. There are several components of the photosynthetic apparatus
that are actively involved in NPQ. Apart from the LHCs, and the xanthophyll cycle,
the Photosystem II Subunit S (PsbS) is a 22-kDa integral membrane protein that is
essential for the response of the photosynthetic apparatus to high-light and it is
activated by the protonation of key lumen-exposed glutamate residues. Atomistic
details on its involvement in NPQ remain still a mystery. However, It is widely
accepted that NPQ (qE) is co-regulated by low lumen pH and ion fluxes (K+
, Ca2+, Mg2+
, Cl)
in Lumen-Stroma areas. It has also been proposed that the activated PsbS may
strongly interact with some LHCs enabling quenching by providing an alternative
environment for some pigments within these LHCs, or by changing the membrane
organization and dynamics. In this study, PsbS (pdb code 4ri2) is embedded in a lipid
bilayer model membrane (400-500 POPC lipids, 1-palmitoyl-2-oleoyl-sn-glycero-3-
phosphocholine). NPQ conditions are simulated by perturbations in the thylakoid lumen
ionic load. Zeaxanthin (Zea) of the xanthophyll cycle that is produced under NPQ is
also embedded in the membrane. We employ large-scale Molecular Dynamics simulations
to probe the PsbS conformational changes, membrane dynamics, or Zea binding that
activate PsbS. We identify two distinct PsbS forms (active-inactive), in response to
A) the lumen acidification or ion fluxes, and B) the Zea binding, revealing a PsbSNPQ
relation at the atomic scale.
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