Proteasome inhibitor PI31 subunit is a protein that in humans is encoded by the PSMF1 gene.[5][6]
The 26S proteasome is a multicatalytic proteinase complex with a highly ordered structure consisting of two subcomplexes, a 20S core and a 19S regulator. The 20S core is made up of 4 rings of 28 non-identical subunits; 2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits. The 19S regulator is composed of a base, which contains 6 ATPase subunits and 2 non-ATPase subunits, and a lid, which contains up to 10 non-ATPase subunits. Proteasomes are distributed throughout eukaryotic cells at a high concentration and cleave peptides in an ATP/ubiquitin-dependent process in a non-lysosomal pathway. An essential function of a modified proteasome, the immunoproteasome, is the processing of class I MHC peptides. PSMF1 encodes a protein termed PI31 that binds to both 20S and 26S proteasomes. PI31 was originally identified by its ability to inhibit the hydrolysis of small synthetic peptides in a cell free assay, but it stimulates 26S proteasome activity both in vitro and in vivo.[7][8] PI31 can promote the assembly of 26S proteasomes from 19S and 20S sub-particles, and it also serves as an adapter to couple proteasomes with motor proteins to mediate fast axonal transport in neurons.[9] Inactivation of PMSF1 in yeast, plants, fruit flies and mice attenuates protein degradation and stimulates accumulation of ubiquitinated protein aggregates. Furthermore, loss of PSMF1 function in mice causes progressive neuronal dysfunction, synaptic degeneration and eventually neuronal cell death.[10] Alternative transcript variants have been identified for this gene.[6]
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