PSMA4 (Proteasome Subunit Alpha 4) is a gene that encodes the alpha-4 subunit of the 20S proteasome core particle, which forms part of the outer alpha-ring structure of the barrel-shaped 20S complex. As an integral component of the 26S proteasome, PSMA4 participates in ATP- and ubiquitin-dependent protein degradation, a process essential for the regulation of cell cycle progression, signal transduction, and protein quality control. In addition, the 20S core particle can associate with regulatory complexes such as PA28 or PA200 to mediate ubiquitin-independent protein degradation, which plays important roles in pathways including antigen peptide generation and spermatogenesis. Dysregulation of PSMA4 expression or function has been implicated in various pathological conditions, including cancer and neurodegenerative diseases, highlighting its critical role in maintaining cellular proteostasis.