Proteasome 4R67, molecular model. Proteasomes are protein-digesting enzymes. Each enzyme consists of a complex of protein subunits with a catalytic core, where the degradation takes place, and two regulatory subunit caps. The regulatory subunits recognise proteins that have been tagged for destruction by the addition of ubiquitin molecules, and transfer them to the hollow catalytic core. The proteasomal degradation pathway is important in many cellular processes, including cell division and gene expression.