The proteasome is a threonine protease in which the active-site threonine residue is at the N-terminus of the beta subunits. Only three of the seven eukaryotic beta subunits (beta1, beta2 and beta5) retain an intact active site. These subunits are activated following autocatalytic processing of an N-terminal propeptide - propeptide autocleavage and substrate proteolysis utilize closely related mechanisms. (Adapted from PMID:28583440.)