The proteasome is a large, highly conserved protein complex whose main function is to enzymatically degrade target proteins. The eukaryotic proteasome houses six protease sites that are located deep within the barrel-shaped 20S core particle. Each core particle is built from two outer rings formed from seven alpha subunits, and two inner rings formed from seven beta subunits. The N-termini of the alpha subunit rings on either end of the core particle form a gate which can open to allow passage of target proteins into the proteasome core. This animation shows the structure of the eukaryotic proteasome. The lid structure is approximated from cryo-EM studies, and the core particle and regulatory particle were created using crystal structure data.