Proteasome activator complex subunit 1 is a protein that in humans is encoded by the PSME1 gene. PSME1 is the alpha subunit of the 11S regulator, one of the two 11S subunits that is induced by gamma-interferon. The PSME1 protein plays a crucial role in the assembly of the immunoproteasome and is indispensable for effective antigen processing. As part of the PA28 activator complex, PSME1 collaborates with PSME2 to actively enhance the production of class I-binding peptides by altering the proteolytic cleavage pattern of the proteasome.