EVIDENCE FOR ENHANCED UBIQUITIN-MEDIATED PROTEOLYSIS OF THE CHICKEN PROGESTERONE-RECEPTOR BY PROGESTERONE

Genomic actions of progesterone are mediated via A and B isoforms of t he progesterone receptor (PR). One major factor controlling PR level i s progesterone causing negative autoregulation (down-regulation) of th e receptor protein. In this work we studied the mechanism whereby prog esterone exerts its effects on PR level in the chicken oviduct. We fou nd that progesterone does not markedly regulate PR mRNA expression. Fu rthermore, we demonstrate here for the first time that PR is a target for ubiquitylation and that the proportion of ubiquitylated PR is incr eased by progesterone treatment. Our data suggest that ligand-induced down-regulation of PR involves enhanced degradation of receptor protei n by ubiquitin-proteasome system in vivo.