Prolidase, also known as Xaa-Pro dipeptidase or peptidase D (PEPD), is a 
ubiquitously expressed cytosolic enzyme that hydrolyzes dipeptides with proline 
or hydroxyproline at the carboxyl terminus. In this article, however, we 
demonstrate that PEPD directly binds to and activates epidermal growth factor 
receptor (EGFR), leading to stimulation of signaling proteins downstream of 
EGFR, and that such activity is neither cell-specific nor dependent on the 
enzymatic activity of PEPD. In line with the pro-survival and pro-proliferation 
activities of EGFR, PEPD stimulates DNA synthesis. We further show that PEPD 
activates EGFR only when it is present in the extracellular space, but that PEPD 
is released from injured cells and tissues and that such release appears to 
result in EGFR activation. PEPD differs from all known EGFR ligands in that it 
does not possess an epidermal growth factor (EGF) motif and is not synthesized 
as a transmembrane precursor, but PEPD binding to EGFR can be blocked by EGF. In 
conclusion, PEPD is a ligand of EGFR and presents a novel mechanism of EGFR 
activation.