The prostate is a relatively homogeneous tissue that is highly specialized in synthetic and secretory functions. The frequency of malignant growth explains its great clinical significance. We used here a combination of subcellular fractionation, 1-DE (one-dimensional gel electrophoresis) protein separation and mass spectrometry, to establish a prostate protein expression profile in mice. Analysis of proteins present in cytosolic (C) and membrane (P) prostate fractions led to the identification of 619 distinct proteins. A majority of abundant proteins were found to compose the metabolism and protein synthesis machinery. Those identified also correspond to known endoplasmic reticulum and Golgi residents, chaperones and anterograde cargos. They included a series of proteins involved in exocytic/endocytic trafficking. Among the signaling proteins, we identified the ubiquitin-like peptides smt3. We showed that both free small ubiquitin-related modifier SUMO-2/3 and SUMO-1 levels are subject to tight control by the androgen 5alpha-dihydrotestosterone (DHT). By contrast with SUMO-2/3, free SUMO-1 peptides are particularly abundant in the prostate when compared with other tissues. Therefore, we report prostate protein expression profiles of cytosolic and membrane fractions in mice. Our data suggest that the identified free SUMO peptides play an important role in this secretory tissue.