beta_helix Staff Lv 1
This tiny domain is an independently-folding piece of a larger ubiquitin ligase protein, responsible for tagging proteins with ubiquitin (also a protein) thereby marking them for death.
This particular ubiquitin ligase is capable of binding and tagging viral proteins - but this turns out to be a bad thing, as certain viruses (like Ebola) need to have ubiquitin attached to one of their proteins in order to break free of the host cell that made them.