On the evolution of chaperones and cochaperones and the expansion of proteomes across the Tree of Life

Across the Tree of Life (ToL), the complexity of proteomes varies widely. Our systematic analysis depicts that from the simplest archaea to mammals, the total number of proteins per proteome expanded -200-fold. Individual proteins also became larger, and multidomain proteins expanded -50-fold. Apart from duplication and divergence of existing proteins, completely new proteins were born. Along the ToL, the number of different folds expanded -5fold and fold combinations -20-fold. Proteins prone to misfolding and aggregation, such as repeat and beta-rich proteins, proliferated -600-fold and, accordingly, proteins predicted as aggregationprone became 6-fold more frequent in mammalian compared with bacterial proteomes. To control the quality of these expanding proteomes, core chaperones, ranging from heat shock proteins 20 (HSP20s) that prevent aggregation to HSP60, HSP70, HSP90, and HSP100 acting as adenosine triphosphate (ATP)-fueled unfolding and refolding machines, also evolved. However, these core chaperones were already available in prokaryotes, and they comprise -0.3% of all genes from archaea to mammals. This challenge- roughly the same number of core chaperones supporting a massive expansion of proteomes-was met by 1) elevation of messenger RNA (mRNA) and protein abundances of the ancient generalist core chaperones in the cell, and 2) continuous emergence of new substrate-binding and nucleotide-exchange factor cochaperones that function cooperatively with core chaperones as a network.

Automated summary

As life evolved along the Tree of Life, the number and size of proteins in proteomes from archaea to mammals increased significantly, with new proteins and combinations of protein folds arising. To maintain quality control, core chaperones and emerging co-chaperones cooperatively evolved into a network to support the massive expansion of proteomes.