Identification of Estrogen Receptor β as a SUMO-1 Target Reveals a Novel Phosphorylated Sumoylation Motif and Regulation by Glycogen Synthase Kinase 3β

SUMO conjugation has emerged as a dynamic process in regulating protein function. Here we identify estrogen receptor beta (ER beta) to be a new target of SUMO-1. ER beta SUMO-1 modification occurs on a unique nonconsensus sumoylation motif which becomes fully competent upon phosphorylation of its contained serine residue, which provides the essential negative charge for sumoylation. This process is further regulated by phosphorylation of additional adjacent serine residues by glycogen synthase kinase 3 beta (GSK3 beta), which maximizes ER beta sumoylation in response to hormone. SUMO-1 attachment prevents ER beta degradation by competing with ubiquitin at the same acceptor site and dictates ER beta transcriptional inhibition by altering estrogen-responsive target promoter occupancy and gene expression in breast cancer cells. These findings uncovered a novel phosphorylated sumoylation motif (pSuM), which consists of the sequence psi KXS (where psi represents a large hydrophobic residue) and which is connected to a GSK3-activated extension that functions as a SUMO enhancer. This extended pSuM offers a valuable signature to predict SUMO substrates under protein kinase regulation.