Arabidopsis CURVATURE THYLAKOID1 Proteins Modify Thylakoid Architecture by Inducing Membrane Curvature

Abstract

Chloroplasts of land plants characteristically contain grana, cylindrical stacks of thylakoid membranes. A granum consists of a core of appressed membranes, two stroma-exposed end membranes, and margins, which connect pairs of grana membranes at their lumenal sides. Multiple forces contribute to grana stacking, but it is not known how the extreme curvature at margins is generated and maintained. We report the identification of the CURVATURE THYLAKOID1 (CURT1) protein family, conserved in plants and cyanobacteria. The four Arabidopsis thaliana CURT1 proteins (CURT1A, B, C, and D) oligomerize and are highly enriched at grana margins. Grana architecture is correlated with the CURT1 protein level, ranging from flat lobe-like thylakoids with considerably fewer grana margins in plants without CURT1 proteins to an increased number of membrane layers (and margins) in grana at the expense of grana diameter in overexpressors of CURT1A. The endogenous CURT1 protein in the cyanobacterium Synechocystis sp PCC6803 can be partially replaced by its Arabidopsis counterpart, indicating that the function of CURT1 proteins is evolutionary conserved. In vitro, Arabidopsis CURT1A proteins oligomerize and induce tubulation of liposomes, implying that CURT1 proteins suffice to induce membrane curvature. We therefore propose that CURT1 proteins modify thylakoid architecture by inducing membrane curvature at grana margins.

Light-harvesting Complexes (LHCs) Cluster Spontaneously in Membrane Environment Leading to Shortening of Their Excited State Lifetimes

“…Indeed, it has been shown that the curvature represents a stress that induces modification in some membrane proteins (36). Interestingly, such a degree of curvature is similar to that of the grana margin of the thylakoid membrane (37). These regions are particularly important for the regulation of association of LHCII with the two photosystems (38) and for the disassembling/repairing of photosystem II (39).…”

Singlet oxygen- and EXECUTER1-mediated signaling is initiated in grana margins and depends on the protease FtsH2

“…1D). The 10,000 × g pellet showed a Chl a/b ratio of 2.5, was highly enriched in the D1 and D2 RC proteins of PSII, was depleted in the PSI-specific PsaF protein and ATPase, and also contained the marker of grana margins, CURT1A (14). The 40,000 × g pellet was similarly enriched in D1 and D2 and depleted in PsaF and ATPase as the 10,000 × g pellet, had a slightly lower Chl a/b ratio of 2.2, but did not contain CURT1A.…”

“…Almost all of CURT1A was confined to the 10,000 × g pellet. CURT1A was identified immunologically with a CURT1A antiserum that recognized the 11,000-kDa polypeptide in WT but not in the quadruple curt1abcd mutant (14). Throughout the fractionation procedure, starting with chloroplast isolation, SIGMAFAST Protease Inhibitor was added to all buffers (1 tablet per 300 mL buffer).…”

Quantitative Circadian Phosphoproteomic Analysis of Arabidopsis Reveals Extensive Clock Control of Key Components in Physiological, Metabolic, and Signaling Pathways

“…These include diurnally-driven phospho-oscillations of some chloroplast proteins in response to light/dark cycles, and circadian control of the phosphorylation state of the D1 photosystem II reaction center protein (106 -108). We have identified circadian control of the phosphorylation state of three thylakoid membrane proteins: Two light harvesting complex proteins of Photosystem I (PSI) LHCA2 and LHCA4, and CURVATURE THYLAKOID1B (CURT1B)/TMP14 protein, a modifier of thylakoid membrane architecture (109). Previous studies identified light/dark dependent phosphorylation of CURT1B/TMP14 at T65 (110, 111), which we confirm and also show to be phased early in the subjective day (LL29) (supplemental Table S3).…”