Alpha-ring Independent Assembly of the 20S Proteasome

Archaeal proteasomes share many features with their eukaryotic counterparts and serve as important models for assembly. Proteasomes are also found in certain bacterial lineages yet their assembly mechanism is thought to be fundamentally different. Here we investigate alpha-ring formation using recombinant proteasomes from the archaeon Methanococcus maripaludis. Through an engineered disulfide cross-linking strategy, we demonstrate that double alpha-rings are structurally analogous to half-proteasomes and can form independently of single alpha-rings. More importantly, via targeted mutagenesis, we show that single alpha-rings are not required for the efficient assembly of 20S proteasomes. Our data support updating the currently held "alpha-ring first" view of assembly, initially proposed in studies of archaeal proteasomes, and present a way to reconcile the seemingly separate bacterial assembly mechanism with the rest of the proteasome realm. We suggest that a common assembly network underpins the absolutely conserved architecture of proteasomes across all domains of life.