The 26S proteasome is the main machine for selective protein destruction in the eukaryotic cell. Its structure and its mechanism for recognizing substrates are strikingly conserved from yeast to humans. In plants, however, the number of genes encoding proteasome subunits varies widely, and until now it was unclear how these duplications affect regulation. Sitting alongside this paper in the archive are studies of the cellular orchestra of genes, the cell's motors, and the stochastic symphony of expression; all of them touch in one way or another on the dynamics of gene networks, but this article stands out because it is the first to systematically trace the evolution of proteasome genes and their regulatory elements across the entire green plant lineage.
The authors assembled the genomes of 18 Viridiplantae species, from the unicellular alga Chlamydomonas reinhardtii to flowering plants, and identified all genes for 26S proteasome subunits by homology search. For each subunit family they built alignments and phylogenetic trees, then reconciled them with the species tree to reconstruct the history of duplications. They scanned proteasome gene promoters for enrichment of short DNA motifs, comparing their frequency near the transcription start site with the genome-wide background. They also drew on post-translational modification data, predictions of intrinsically disordered regions, and calculations of electrostatic potential. To test the role of telomere-binding proteins, they ran electrophoretic mobility shift assays, analyzed Arabidopsis mutants, and reanalyzed ChIP-seq data.
In 16 of the 18 species, duplicated proteasome genes turned up, and the duplication patterns were species-specific and mostly recent. Ancient whole-genome duplications had almost no effect on the expansion of the family, whereas recent ones, in mosses or legumes for instance, left their mark. At the protein level, selection was purifying: only 4 cases out of 886 showed signs of positive selection. Differences between paralogs came down to subtle tweaks in the electrostatic potential of the surface and shifts in phosphorylation sites. Regulation diverged far more: paralogs responded differently to stress, and this trend can be traced from mosses all the way to flowering plants. In the promoters of proteasome genes in angiosperms, the PRCE motif is enriched; it is recognized by NAC53/78 factors and, as it turned out, by TRB1/3/5 proteins as well. The trb4 trb5 mutants proved hypersensitive to a proteasome inhibitor, and induction of proteasome genes was disrupted in them. Finally, a coevolution analysis revealed a network of 386 proteins evolving in concert with the proteasome and enriched for the same regulatory motifs.
The work shows for the first time that in plants a conserved protein degradation machine evolves not by changing the proteins themselves but by rewiring its regulatory logic. This sets it apart from neighboring studies that focused on individual aspects of gene expression or gene network dynamics but did not connect duplications to promoter evolution. The discovery of the role played by TRB proteins in proteasome regulation adds a new layer to our understanding of how the cell coordinates proteostasis with other processes, including the cell cycle and photosynthesis.
Further research could test exactly how TRB proteins interact with NAC53/78 over time and what chromatin rearrangements they trigger. It would also be interesting to find out whether TRB proteins themselves are subject to degradation by the proteasome, which would close the feedback loop. Comparisons with other eukaryotes, especially animals, which also have subunit duplications, would help clarify how universal the strategy of regulatory diversification is.
The results matter for plant biology, evolutionary developmental biology, and the study of the ubiquitin-proteasome system. They may also be useful in plant biotechnology, where fine-tuning proteostasis matters for stress resistance.
Test the functional significance of the species-specific motifs found in proteasome gene promoters across different species. Determine which transcription factors recognize these motifs outside the angiosperms.
Plant biology still has an open question: how do conserved cellular machines adapt to diverse environmental conditions? This work links gene duplications to the evolution of regulatory networks, a key problem in evolutionary developmental biology. It also sheds light on the coordination of proteostasis and energy metabolism, which matters for understanding plant stress responses.
In Arabidopsis, the promoters of proteasome genes contain a motif that also shows up in telomeres, the end sections of chromosomes. Proteins that normally guard the ends of chromosomes turned out to be capable of switching on proteasome genes.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.