We studied the participation of the ubiquitin proteasome system (UPS) in spermadhesin release during in vitro capacitation (IVC) of domestic boar spermatozoa. At ejaculation, boar spermatozoa acquire low molecular weight (8-16 kDa) seminal plasma proteins, predominantly spermadhesins, aggregated on the sperm surface. Due to their arrangement, such aggregates are relatively inaccessible to antibody labeling. As a result of de-aggregation and release of the outer layers of spermadhesins from the sperm surface during IVC, antibody labeling becomes feasible in the capacitated spermatozoa. In vivo, the capacitation-induced shedding of spermadhesins from the sperm surface is associated with the release of spermatozoa from the oviductal sperm reservoir. We took advantage of this property to perform image-based flow cytometry to study de-aggregation and shedding of boar spermadhesins (AQN, AWN, PSP protein families) and boar DQH (BSP1) sperm surface protein which induces higher fluorescent intensity in capacitated vs. ejaculated spermatozoa. Addition of a proteasomal inhibitor (100 µM MG132) during IVC significantly reduced fluorescence intensity of all studied proteins (P<0.05) compared to vehicle control IVC. Western blot detection of spermadhesins did not support their retention during IVC with proteasomal inhibition (P>0.99) but showed the accumulation of DQH (P=0.03) during IVC, compared to vehicle control IVC. Our results thus demonstrate that UPS participates in the de-aggregation of spermadhesins and DQH protein from the sperm surface during capacitation, with a possible involvement in sperm detachment from the oviductal sperm reservoir and/or sperm-zona pellucida interactions.