Low-pH induced reversible reorganizations of chloroplast thylakoid membranes — As revealed by small-angle neutron scattering

Abstract

Energization of thylakoid membranes brings about the acidification of the lumenal aqueous phase, which activates important regulatory mechanisms. Earlier Jajoo and coworkers (2014 FEBS Lett. 588:970) have shown that low pH in isolated plant thylakoid membranes induces changes in the excitation energy distribution between the two photosystems. In order to elucidate the structural background of these changes, we used small-angle neutron scattering on thylakoid membranes exposed to low p 2 H (pD) and show that gradually lowering the p 2 H from 8.0 to 5.0 causes small but well discernible reversible diminishment of the periodic order and the lamellar repeat distance and an increased mosaicity -similar to the effects elicited by light-induced acidification of the lumen. Our data strongly suggest that thylakoids dynamically respond to the membrane energization and actively participate in different regulatory mechanisms. Keywords: chloroplast thylakoid membranes, lamellar repeat distance, low pH and p 2 H, smallangle neutron scattering (SANS) Highlights:1. Thylakoid membranes exposed to low p 2 H studied by small-angle neutron scattering 2. Acidification causes reversible shrinkage and diminished lamellar order 3. SANS changes induced by low pH resemble those due to light-induced lumenal acidification Abbreviations: p 2 H (pD), deuterium analogue of pH; NPQ, non-photochemical quenching; qE, the energy-dependent component of NPQ; ∆µ H + , transmembrane electrochemical potential gradient; PSI, photosystem I; PSII, photosystem II; LET, linear electron transport; CD, circular dichroism; SANS, small-angle neutron scattering; q, scattering vector; I, intensity; q*, center position of the Bragg peak; RD, repeat distance; φ, azimuthal angle; I(φ), angular dependency of the scattering intensity; FWHM, full width at half maximum 3

Lipid polymorphism in chloroplast thylakoid membranes – as revealed by 31P-NMR and time-resolved merocyanine fluorescence spectroscopy

“…The energy-dependent NPQ depends on the transmembrane ∆pH both via the PsbS protein and the XC. Low-pH and transmembrane-∆pH induced structural changes have been thoroughly documented in the literature 62 – 69 . Recently it has been shown using SANS that low pH induces significant variations in the periodic organization of chloroplast thylakoid membranes 69 .…”

“…Low-pH and transmembrane-∆pH induced structural changes have been thoroughly documented in the literature 62 – 69 . Recently it has been shown using SANS that low pH induces significant variations in the periodic organization of chloroplast thylakoid membranes 69 . The observed reorganizations, shrinking of the lamellar repeat distances and the diminishment of the periodic order of the thylakoid membranes closely resembled the light-induced, ∆pH-dependent reorganizations in isolated thylakoid membranes 70 .…”

How to Measure Grana – Ultrastructural Features of Thylakoid Membranes of Plant Chloroplasts

“…High-resolution TEM, although requiring sample fixation, remains the most favored method to study the grana morphology due to the nanometer length scale of the grana structural details. The thylakoid ultrastructure might also be assessed using small-angle scattering methods that enable a noninvasive analysis of high volume samples (Ünnep et al, 2017;Jakubauskas et al, 2019;Ünnep et al, 2020;Zsiros et al, 2020;Jakubauskas et al, 2021). Only periodic membrane attributes can be registered, while no information on grana diameter or details of membrane connections could be revealed.…”

Thylakoid membrane reorganizations revealed by small-angle neutron scattering of Monstera deliciosa leaves associated with non-photochemical quenching

“…In excess light, sustained acidification of the lumen is sensed by PsbS protein in plants and triggers the qE through protonation of PSII proteins, it also activates the xanthophyll cycle [11,71,72]. In general, low pH and the light-induced transmembrane ΔpH have been shown to induce structural changes at different levels of structural complexity, at the microscopic levels affecting the distribution of protein complexes (for reviews see [22,73], the lipid phases [74,75], and assemblies at higher levels of the membrane organization [17,18,20,21,49,[76][77][78]. While structural changes appear to be ubiquitous in oxygenic photosynthetic organisms, NPQ is not.…”