Low pH modulates the macroorganization and thermal stability of PSII supercomplexes in grana membranes

Abstract

Protonation of the lumen-exposed residues of some photosynthetic complexes in the grana membranes occurs under conditions of high light intensity and triggers a major photoprotection mechanism known as energy dependent nonphotochemical quenching. We have studied the role of protonation in the structural reorganization and thermal stability of isolated grana membranes. The macroorganization of granal membrane fragments in protonated and partly deprotonated state has been mapped by means of atomic force microscopy. The protonation of the photosynthetic complexes has been found to induce large-scale structural remodeling of grana membranes-formation of extensive domains of the major light-harvesting complex of photosystem II and clustering of trimmed photosystem II supercomplexes, thinning of the membrane, and reduction of its size. These events are accompanied by pronounced thermal destabilization of the photosynthetic complexes, as evidenced by circular dichroism spectroscopy and differential scanning calorimetry. Our data reveal a detailed nanoscopic picture of the initial steps of nonphotochemical quenching.

Figure 1

(A) Representative contact mode AFM height image of BBY^pH7.8 membrane. (B) Height profile from dashed line in (A), showing the two height levels, marked with arrows.

Figure 2

(A) Representative higher-resolution contact mode AFM height image of a BBY^pH7.8 membrane. (B) Vertical deflection image from (A). (C) Height profile from dashed line in A, showing the height of two protrusions.

Figure 3

The 77 K fluorescence emission spectra of BBY^pH7.8 (black line) and BBY^pH5.2 (gray line) membrane fragments recorded upon excitation at 436 nm.

Figure 4

(A) Representative contact mode AFM height image of a BBY^pH5.2 membrane. (B) Height profile from dashed line in (A), showing the two height levels, marked with arrows.

Figure 5

(A) Representative contact mode AFM height image of a BBY^pH5.2 membrane. (B) Vertical deflection image from (A). (C) Height profile from dashed line in A.

Figure 6

(A) Typical CD spectra of BBY^pH7.8 (A, black line) and BBY^pH5.2 (A, red line) membranes. Selected thermal dependences of the 665/680 nm (B) and 610/646 nm (C) CD bands recorded for BBY^pH7.8 (solid black squares) and BBY^pH5.2 membranes (open red circles). The data points are fitted either with a bisigmoidal (B and C, black lines) or sigmoidal (B and C, red lines) function.

Figure 7

(A) DSC thermograms of BBY^pH7.8 (A, black line) and BBY^pH5.2 (A, gray line) membranes. Scanning rate 1°C/min. The mathematical deconvolution (thin lines) of the thermograms is shown in (B) (BBY^pH7.8) and (C) (BBY^pH5.2).