This animation shows how proteins in the cell are tagged for disposal by ubiquitin and then degraded by the proteasome.

Cells tag misfolded or unneeded proteins with a molecule called ubiquitin. The animation illustrates how ubiquitin is attached to an example protein, ataxin-1, with the help of the ubiquitin carrier enzyme and ubiquitin ligase. Once tagged, ataxin-1 is recognized by a structure called the proteasome, which breaks the protein down into reusable amino acids.

The animation also shows how a mutant form of ataxin-1 cannot be broken down by the proteasome. These mutant proteins accumulate over time and can lead to diseases such as spinocerebellar ataxia type 1 (SCA1).

This animation is a clip from a 2003 Holiday Lecture Series, Learning From Patients: The Science of Medicine. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different parts of the ubiquitin-proteasome system.