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Chloroplast SRP43 autonomously protects chlorophyll biosynthesis proteins against heat shock

Abstract

The assembly of light-harvesting chlorophyll-binding proteins (LHCPs) is coordinated with chlorophyll biosynthesis during chloroplast development. The ATP-independent chaperone known as chloroplast signal recognition particle 43 (cpSRP43) mediates post-translational LHCP targeting to the thylakoid membrane and also participates in tetrapyrrole biosynthesis (TBS). How these distinct actions of cpSRP43 are controlled has remained unclear. Here, we demonstrate that cpSRP43 effectively protects several TBS proteins from heat-induced aggregation and enhances their stability during leaf greening and heat shock. While the substrate-binding domain of cpSRP43 is sufficient for chaperoning LHCPs, the stabilization of TBS clients requires the chromodomain 2 of the protein. Strikingly, cpSRP54—which activates cpSRP43’s LHCP-targeted function—inhibits the chaperone activity of cpSRP43 towards TBS proteins. High temperature weakens the interaction of cpSRP54 with cpSRP43, thus freeing cpSRP43 to interact with and protect the integrity of TBS proteins. Our data indicate that the temperature sensitivity of the cpSRP43–cpSRP54 complex enables cpSRP43 to serve as an autonomous chaperone for the thermoprotection of TBS proteins.

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Fig. 1: cpSRP43 is required for the stabilization of GluTR, CHLH and GUN4.
Fig. 2: Physical interactions of cpSRP43 with CHLH and GUN4.
Fig. 3: cpSRP43 effectively chaperones TBS proteins.
Fig. 4: The chaperone activity of cpSRP43 towards TBS proteins requires both its SBD and at least one of the two C-terminal CDs.
Fig. 5: The CD2 domain of cpSRP43 is indispensable for the stability of TBS proteins in planta.
Fig. 6: cpSRP54 inhibits the chaperone function of cpSRP43 for TBS proteins.
Fig. 7: Model for the dual participation of cpSRP43 in Chl synthesis and LHCP transport in plants.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information. Source data are provided with this paper.

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Acknowledgements

We thank D. Schünemann (Ruhr-Universität Bochum) for providing us the chaos and ffc mutants, the GST–cpSRP43 expression vector, and antibodies against cpSRP43 and cpSRP54, and P. Hardy for critical reading of the manuscript. This work was supported by a grant from the Chinese Scholarship Council to S.J., grant nos R35 GM136321 and DOE.DE-SC0020661 to A.S. and S.S., and grants from the Deutsche Forschungsgemeinschaft to B.G. (nos FOR2092, GR 936/18-1 and SFB TRR175, subproject C04) and to P.W. (no. WA 4599/2-1).

Author information

Authors and Affiliations

Authors

Contributions

B.G. and P.W. designed the experiments. S.J. and A.S. performed the experiments. S.J., A.S., S.S., B.G. and P.W. analysed the data. B.G. and P.W. wrote the manuscript. S.J., A.S. and S.S. gave critical comments and revisions to the manuscript.

Corresponding authors

Correspondence to Bernhard Grimm or Peng Wang.

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The authors declare no competing interests.

Additional information

Peer review information Nature Plants thanks Briardo Llorente, Tatsuru Masuda and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 cpSRP54 deficiency does not compromise the abundance of GluTR, CHLH, and GUN4.

a, Phenotype of 18-day-old cpsrp54 knock-out mutant (ffc) and wild-type (Col-0) in long-day (18 h light/ 6 h dark). Scale bar, 1 cm. b, Steady-state levels of TBS proteins in the wild-type and ffc were detected by immunoblotting using the indicated antibodies. Ponceau S stained large subunit of RubisCo (RbcL) and Actin are shown as loading controls. c, Semiquantitative analysis with Image J software (NIH) of the immunoblots in (b). The relative amounts of GluTR, CHLH and GUN4 in ffc were normalized to the levels in Col-0. The data are plotted as means ± s.d. (n = 3 independent biological replicates). The statistical analyses were performed using two-tailed Student’s t-tests. No asterisk indicates no significant differences compared to protein levels in Ler-0: * P < 0.05, **P < 0.01.

Source data

Extended Data Fig. 2 Physical interaction of cpSRP43 with CHLH and GUN4.

a, Immunoblotting analyses of protein extracts after the BiFC assay (shown in Fig. 2a) confirmed expression of cpSRP43-YFPn, GUN4-YFPn (c-Myc antibody) and YFPc tagged CHLM (GFP antibody) and CHLH (CHLH antibody). b, Immunoblotting analyses confirmed expression of cpSRP43-YFPn and GUN4-YFPc shown in Fig. 2a. Three biological repeats have been performed with similar results.

Source data

Extended Data Fig. 3 In vitro enzymatic assays of MgCh.

a, Time course curves of the MgCh enzymatic assay with fluorescence detection of MgP (Ex 416 nm/Em 595 nm). The data are plotted as mean ± s.d. (n = 3 independent experiments). b, Recombinant proteins used in the assay were stained by Coomassie Brilliant Blue.

Source data

Extended Data Fig. 4 Short-term heat stress does not alter the transcriptional levels of GluTR, CHLH, GUN4, cpSRP43 and cpSRP54.

Relative mRNA levels of indicated genes in 18-day-old Ler-0, chaos and cpSRP43-OX were measured prior to and after 2 h heat treatment at 42 °C. Gene expression was calculated relative to the Ler-0 and SAND was used as the reference gene. N.D., not detected. The data are plotted as mean ± s.d. (n = 3 independent biological repeats). The small open circles represent the individual values. Letters above histograms indicate significant differences as determined by two-way ANOVA with Tukey test (P < 0.05).

Source data

Extended Data Fig. 5 Measurement of the binding affinity of cpSRP43 with GUN4 using microscale thermophoresis (MST).

The fluorophore-labelled GUN4 were used at constant concentration (20 nM), while the concentration of the non-labelled wild-type (a) or truncated cpSRP43 (b-e) were varied between 3.05 nM and 100 μM. N.D., not detectable. The data are plotted as mean ± s.d. (n = 3 independent experiments).

Source data

Extended Data Fig. 6 Accumulation of Chl and Chl precursors in various chaos complementation lines.

HPLC analyses of Chl (a), Proto, MgP and MgPMME (b) and Pchlide (c) in the seedlings of 18-day-old Ler, chaos and a set of chaos complementation lines. FW, fresh weight. The data are plotted as mean ± s.d. (n = 3 independent biological repeats), the small open circles represent the individual values. Letters above histograms indicate significant differences as determined by one-way ANOVA with Tukey test (P < 0.05).

Source data

Extended Data Fig. 7 The cpSRP54M peptide stimulates the chaperone activity of cpSRP43 cysless towards LHCP.

The turbidity of 1 µM LHCP diluted out of 8 M urea was measured in the presence of cysteine-mutated cpSRP43 (cpSRP43 C175A/C297S) with (red line) or without (blue line) 50 μM cpSRP54M peptide present. The turbidity at 360 nm was normalized to the value at 0 μM cpSRP43, and the lines are fits of the data to Eq. 1 in the Methods, and the obtained Ksol values were 2 ± 0.4 μM and 0.03 ± 0.09 μM for apo cpSRP43 and cpSRP43 plus cpSRP54M peptide, respectively. This experiment was performed for one time, the similar results have been obtained in the previous publication27.

Source data

Extended Data Fig. 8 Comparison of cpSRP43 chaperone activity towards GUN4 and GluTR in the presence of cpSRP54M peptide, cpSRP54M domain, and full-length mature cpSRP54.

Heat-induced aggregation of GluTR (a) and GUN4 (b) was monitored in the presence of apo cpSRP43 (blue lines in each panel) and compared to cpSRP43 with 50 μM cpSRP54M peptide (red lines in the left panels), 20 μM cpSRP54M domain (red lines in the middle panels), or 10 μM full length (FL) mature cpSRP54 (red lines in the right panels). Additional experiments were carried out with cpSRP43 and each cpSRP54 variant in the absence of either client (grey lines in each panel). The turbidity at 360 nm was normalized to the value at 0 μM cpSRP43 under each condition, plotted as mean ± s.e. (n = 3 independent experiments), and fit to Eq. 1 in the Methods. Data and Ksol values for apo cpSRP43 are the same as in Figs. 3a and c. Ksol values for cpSRP43 in the presence of cpSRP54M variants were not obtained, as no protection was observed.

Source data

Extended Data Fig. 9 cpSRP54 deficiency does not compromise Chl biosynthesis during heat treatment.

HPLC analyses of Chl (a), MgP and MgPMME (b) and Pchlide (c) in 18-day-old Col-0 and ffc prior to and after 2 or 6 h heat treatment at 42 °C. DW, dry weight. The data are plotted as mean ± s.d. (n = 3 independent biological repeats). The small open circles represent the individual values. Letters above histograms indicate significant differences as determined by two-way ANOVA with Tukey test (P< 0.05).

Source data

Extended Data Fig. 10 Heat treatment promotes the binding affinity of cpSRP43 with GluTR, CHLH and GUN4.

a, Co-immunoprecipitation assay to determine the changes in the interaction of cpSRP43 with GluTR, CHLH and GUN4 prior to or after heat treatment at 42 °C for 2 hours (h). Total chloroplast extracts from wild type (Ler-0, used as negative control) and transgenic Arabidopsis lines overexpressing cpSRP43-FLAG were incubated with anti-FLAG affinity beads. cpSRP43 interaction partners were detected by immunoblotting analyses using the indicated antibodies. b-d, In vitro His pull-down assay showing the enhanced binding affinity of GST-cpSRP43 with His-tagged GluTR (b), CHLH (c) and GUN4 (d) at high temperature. Recombinant purified His-tagged GluTR, CHLH and GUN4 (5 μM) were used as bait and incubated with GST or GST-cpSRP43 (5 μM) at 15 °C or 30 °C for 1 h. The proteins bound to His-tagged GluTR, CHLH and GUN4 were eluted with the elution buffer containing 250 mM imidazole. Input and elution fractions were analysed by immunoblot analyses using the indicated antibodies. Two independent biological repeats have been performed with similar results.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1 and 2 and Table 1.

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Unprocessed western blots and gels, and statistical source data.

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Unprocessed western blots and gels, and statistical source data.

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Unprocessed western blots and gels, and statistical source data.

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Unprocessed western blots and gels.

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Ji, S., Siegel, A., Shan, So. et al. Chloroplast SRP43 autonomously protects chlorophyll biosynthesis proteins against heat shock. Nat. Plants 7, 1420–1432 (2021). https://doi.org/10.1038/s41477-021-00994-y

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