P23

p23 is a small, 23-kilodalton protein that functions as a co-chaperone of the Hsp90 chaperone system in eukaryotic cells. In humans it is encoded by the PTGES3 gene and is commonly referred to as p23 or Hsp90 co-chaperone p23. The protein is predominantly cytosolic and can shuttle to the nucleus in response to cellular stress.

As an Hsp90 co-chaperone, p23 binds to Hsp90 and modulates its ATPase activity, helping stabilize late-stage Hsp90–client complexes. This regulation slows ATP hydrolysis by Hsp90, extending the time available for client protein folding and maturation.

p23 participates in the maturation of numerous Hsp90 client proteins, especially steroid hormone receptors such as glucocorticoid, estrogen, and progesterone receptors, as well as certain kinases. It acts as an adaptor that helps deliver clients to Hsp90 and stabilizes their mature conformations, supporting proper signaling and function.

Clinically, altered p23 expression has been observed in various cancers and neurodegenerative diseases. Because p23 influences the stability of oncogenic signaling pathways via Hsp90, it is investigated as a potential biomarker and therapeutic target, often in conjunction with Hsp90 inhibitors that disrupt the chaperone cycle.