MET1 Is a Thylakoid-Associated TPR Protein Involved in Photosystem II Supercomplex Formation and Repair in Arabidopsis

Abstract

ORCID ID: 0000-0001-9536-0487 (K.J.v.W.)Photosystem II (PSII) requires constant disassembly and reassembly to accommodate replacement of the D1 protein. Here, we characterize Arabidopsis thaliana MET1, a PSII assembly factor with PDZ and TPR domains. The maize (Zea mays) MET1 homolog is enriched in mesophyll chloroplasts compared with bundle sheath chloroplasts, and MET1 mRNA and protein levels increase during leaf development concomitant with the thylakoid machinery. MET1 is conserved in C3 and C4 plants and green algae but is not found in prokaryotes. Arabidopsis MET1 is a peripheral thylakoid protein enriched in stroma lamellae and is also present in grana. Split-ubiquitin assays and coimmunoprecipitations showed interaction of MET1 with stromal loops of PSII core components CP43 and CP47. From native gels, we inferred that MET1 associates with PSII subcomplexes formed during the PSII repair cycle. When grown under fluctuating light intensities, the Arabidopsis MET1 null mutant (met1) showed conditional reduced growth, near complete blockage in PSII supercomplex formation, and concomitant increase of unassembled CP43. Growth of met1 in high light resulted in loss of PSII supercomplexes and accelerated D1 degradation. We propose that MET1 functions as a CP43/CP47 chaperone on the stromal side of the membrane during PSII assembly and repair. This function is consistent with the observed differential MET1 accumulation across dimorphic maize chloroplasts.

A chloroplast thylakoid lumen protein is required for proper photosynthetic acclimation of plants under fluctuating light environments

“…S6, S11, and S14). Our data are consistent with published reports that-under growth chamber fluctuating light conditions-tlp18.3, psb27, and met1-2 displayed stunted growth phenotypes (29,40,42,45). Moreover, reduced greenhouse growth is not simply the result of lower PSII efficiency.…”

“…S14 A and B), as previously reported (30,31,42,43). In contrast, these mutants displayed reduced growth and PSII activity in the greenhouse (SI Appendix, Fig.…”

“…4B). In contrast, loss of the PSII supercomplex reassembly factors LQY1, HHL1, or MET1 caused accelerated rates of D1 and D2 degradation under high-light stress (32)(33)(34)42). Because PSII monomer disassembly precedes damaged reaction center degradation and replacement (13)(14)(15)(16), the failure of mph2 mutants to turn over PSII reaction center components under stressful light implicates MPH2 in monomer disassembly.…”

HLB1 Is a Tetratricopeptide Repeat Domain-Containing Protein That Operates at the Intersection of the Exocytic and Endocytic Pathways at the TGN/EE in Arabidopsis

“…Other TPR domain-containing proteins among the 147 in Arabidopsis, including SHORT AND SWOLLEN ROOT1 and MET1, localize to mitochondria and chloroplasts, respectively, consistent with their functions in oxidative stress-mediated root development and in the assembly of the photosynthetic apparatus (Bhuiyan et al, 2015;Zhang et al, 2015). The TPR protein CLUMPED CHLOROPLAST1 (CLMP1) localizes to distinct foci in the cytoplasm.…”

“…TPR domains mediate protein-protein interactions, and proteins containing these domains are part of protein complexes with diverse functions including transcriptional repression, protein import, heat shock, and cell cycle regulation (Blatch and Lässle, 1999;D'Andrea and Regan, 2003). TPR domaincontaining proteins in plants have been implicated in responses to various hormones, assembly of the photosynthetic machinery, plastid partitioning, and root development (Jacobsen et al, 1996;Rosado et al, 2006;Yang et al, 2011;Hu et al, 2014;Bhuiyan et al, 2015). Confocal microscopy of cells from the elongation zone and root hairs of hlb1-1 seedlings expressing HLB1pro:HLB1-GFP revealed that the fusion protein localized to the cytoplasm and decorated distinct and dynamic foci (Figures 6C and 6D; Supplemental Movie 4).…”