Proteasomes and protein conjugation across domains of life

Proteasomes exist in all domains of life and serve to degrade proteins. In eukaryotes, proteins are primarily targeted for proteasomal degradation through the addition of ubiquitin. Similarly, archaea and bacteria modify proteins with Pup and Samps, respectively, and this may also serve as a signal for proteasomal degradation. Like other energy-dependent proteases, proteasomes, which are found across the three domains of life, are self-compartmentalized and important in the early steps of proteolysis. Proteasomes degrade improperly synthesized, damaged or misfolded proteins and hydrolyse regulatory proteins that must be specifically removed or cleaved for cell signalling. In eukaryotes, proteins are typically targeted for proteasome-mediated destruction through polyubiquitylation, although ubiquitin-independent pathways also exist. Interestingly, actinobacteria and archaea also covalently attach small proteins (prokaryotic ubiquitin-like protein (Pup) and small archaeal modifier proteins (Samps), respectively) to certain proteins, and this may serve to target the modified proteins for degradation by proteasomes.