Molecular insights into the unique properties of the blood-circulating proteasome.

Proteasomes are essential for protein degradation and maintaining cellular balance, yet their roles in extracellular fluids are not well understood. Our study investigates the freely circulating proteasome in blood, to uncover its unique molecular characteristics, compared to its intracellular counterparts. Using a transgenic mouse model, mass spectrometry, and biochemical tools, we show that the predominant proteasome in serum is the free uncapped 20S particle, which seems to assemble intracellularly before entering the bloodstream. This serum proteasome is composed of constitutive and immuno subunits and exhibits all three catalytic activities. Moreover, the complex displays distinct post-translational modifications, indicating specialization for extracellular roles, as demonstrated by its enhanced caspase-like activity. We also found that physiological stress significantly upregulates serum 20S proteasome levels, paralleling human data. This research highlights the specialized characteristics of circulating proteasomes, offering new insights into protein turnover in the blood with significant implications for understanding proteostasis beyond the intracellular environment.