Ubiquitin

"Ubiquitin is a small (8.5 kDa) regulatory protein that has been found in almost all tissues (ubiquitously) of eukaryotic organisms. Ubiquitination is a post translational modification (an addition to a protein after it has been made) where ubiquitin is attached to a substrate protein."
http://en.wikipedia.org/wiki/Ubiquitin
The addition of ubiquitin to proteins can signal for their degradation via the proteasome.

This model is based on Human erythrocytic ubiquitin. The C-terminal glycine