Extracellular 20S proteasome secreted via microvesicles can degrade poorly folded proteins and inhibit Galectin‐3 agglutination activity

Proteasomes are major non‐lysosomal proteolytic complexes localized in the cytoplasm and in the nucleus of eukaryotic cells. Strikingly, high levels of extracellular proteasome have also been evidenced in the plasma (p‐proteasome) of patients with specific diseases. Here, we examined the process by which proteasomes are secreted, as well as their structural and functional features once in the extracellular space. We demonstrate that assembled 20S core particles are secreted by cells within microvesicles budding from the plasma membrane. Part of the extracellular proteasome pool is also free of membranes in the supernatant of cultured cells, and likely originates from microvesicles leakage. We further demonstrate that this free proteasome released by cells (cc‐proteasome for cell culture proteasome) possesses latent proteolytic activity and can degrade various extracellular proteins. Both standard (no immune‐subunits) and intermediate (containing some immune‐subunits) forms of 20S are observed. Moreover, we show that galectin‐3, which displays a highly disordered N‐terminal region, is efficiently cleaved by purified cc‐proteasome, without SDS activation, likely after its binding to PSMA3 (α7) subunit through its intrinsically disordered region. As a consequence, galectin‐3 is unable to induce red blood cells agglutination when preincubated with cc‐proteasome. These results highlight potential novel physio‐ and pathologic functions for the extracellular proteasome.