On the Origins of Symmetry and Modularity in the Proteasome Family: Symmetry Transitions are Pivotal in the Evolution and Functional Diversification of Self-Compartmentalizing Proteases.

The proteasome family of proteases comprises oligomeric assemblies of very different symmetry. In different sizes, it features ring-like oligomers with dihedral symmetry that allow the stacking of further rings of regulatory subunits as observed in the modular proteasome system, but also less symmetric helical assemblies. Comprehensive sequence and structural analyses of proteasome homologs reveal a parsimonious scenario of how symmetry may have emerged from a monomeric ancestral precursor and how it may have evolved throughout the proteasome family. The four characterized representatives-ancestral β subunit (Anbu), HslV, betaproteobacterial proteasome homolog (BPH), and the 20S proteasome-are outlasting cornerstones in the family's evolutionary history, each marking a transition in symmetry. This article contextualizes the evolutionary and functional key aspects of these symmetry transitions, explaining how they facilitated the diversification and concurrent evolution of independent proteolytic systems side by side, each with its customized network of auxiliary interactors.