Sperm-specific C-terminal processing of the proteasome PSMA1/α6 subunit

► Ascidian sperm proteasome degrades the egg-coat protein extracellularly during fertilization. ► To explore a transport signal, we investigated the subunits of ascidian sperm proteasome. ► The C-terminal 16 residues of PSMA1/α6 subunit are processed in a sperm-specific manner.