Fast and global reorganization of the chloroplast protein biogenesis network during heat acclimation

Abstract

Photosynthesis is a central determinant of plant biomass production, but its homeostasis is increasingly challenged by heat. Little is known about the sensitive regulatory principles involved in heat acclimation that underly the biogenesis and repair of chloroplast-encoded core subunits of photosynthetic complexes. Employing time-resolved ribosome and transcript profiling together with selective ribosome proteomics, we systematically deciphered these processes in chloroplasts of Chlamydomonas reinhardtii. We revealed protein biosynthesis and altered translation elongation as central processes for heat acclimation and showed that these principles are conserved between the alga and the flowering plant Nicotiana tabacum. Short-term heat exposure resulted in specific translational repression of chlorophyll a-containing core antenna proteins of photosystems I and II. Furthermore, translocation of ribosome nascent chain complexes to thylakoid membranes was affected, as reflected by the increased accumulation of stromal cpSRP54-bound ribosomes. The successful recovery of synthesizing these proteins under prolonged acclimation of non-lethal heat conditions was associated with specific changes of the co-translational protein interaction network, including increased ribosome association of chlorophyll biogenesis enzymes and acclimation factors responsible for complex assembly. We hypothesize that co-translational cofactor binding and targeting might become bottlenecks under heat but become optimized upon heat acclimation to sustain correct co-translational protein complex assembly.

Proteomic identification of the interactome of stalled ribosome nascent chain complexes translating the thylakoid membrane protein D1

“…In accordance with rising evidence for a linkage between the synthesis of chlorophyll and proteins in chloroplasts (Westrich et al, 2021; Trösch et al, 2022), we observed the accumulation of chlorophyll biosynthesis enzymes such as the magnesium protoporphyrin IX methyltransferase (CHLM) and protochlorophyllide reductase (POR) in D1 RNCs. A well-coordinated expression of chlorophyll-binding apoproteins and chlorophyll synthesis is required in the chloroplast to avoid the formation of harmful reactive oxygen species and phototoxic stress due to the accumulation of free, highly photoreactive chlorophyll molecules (Apel and Hirt, 2004; Wang and Grimm, 2015).…”

“…Recent reports indicated that chloroplast ribosomes exhibit a large interaction network that connects the protein synthesis machinery with various metabolic pathways (Westrich et al, 2021; Trösch et al, 2022). Consistently, we identified among the 123 factors enriched in D1 RNCs 55 proteins that could be assigned to different metabolic pathways (Supplemental Table 2).…”

“…In a recent study, it was found that various enzymes, e.g. of carbon, amino acid or chlorophyll metabolism, are associated with isolated chloroplast ribosomes (Westrich et al, 2021; Trösch et al, 2022). In line with this, previous studies have demonstrated a direct association of an enzyme of chlorophyll biosynthesis with chloroplast ribosomes and an enzyme of fatty acid biosynthesis with D1-encoding mRNA, affecting its localized translation (Kannangara et al, 1997; Bohne et al, 2013).…”

Retrograde and anterograde signaling in the crosstalk between chloroplast and nucleus

“…Functional redundancy and significant physical interactions between these proteins have been discovered previously. Using the yeast two hybrid system, bZIP16, bZIP68, and GBF1, were found to form heterodimers (Tano and Woodson, 2022;Trösch et al, 2022). Such heterodimerizations have also been observed between the GBF1, GBF2, and GBF3 proteins (Pesaresi et al, 2007;Toyokuni et al, 2022;Trösch et al, 2022).…”

“…Using the yeast two hybrid system, bZIP16, bZIP68, and GBF1, were found to form heterodimers (Tano and Woodson, 2022;Trösch et al, 2022). Such heterodimerizations have also been observed between the GBF1, GBF2, and GBF3 proteins (Pesaresi et al, 2007;Toyokuni et al, 2022;Trösch et al, 2022). GBF1 was also found to interact with other proteins in a variety of ways, including acting antagonistically with HY5 for the expression of RBCS-1A, acting additively with HY5 to regulate the expression of LHCB1.3 (Pesaresi et al, 2007), and acting antagonistically with MYC2, a bHLH TF, in blue light signaling and regulation of hypocotyl elongation (Tyagi, 2001;Tyagi and Gaur, 2003;Pesaresi et al, 2007).…”