Ubiquitin Combination (IMAGE) Goethe University Frankfurt Caption Modifying the modifier: Crystal structures of ubiquitin (green) and modified ubiquitin (yellow). Modified ubiquitin contains an additional phosphoribosyl group at amino acid position 42. The aligned structures reveal a small, but decisive change in the three dimensional structure of the protein (middle). Credit <i>Cell</i> Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.