HIV Envelope Protein Opens Up (IMAGE) University of California - Davis Caption HIV envelope protein before (orange) and after (green) attaching to a human T cell. Top: The orange shape shows the three-armed HIV envelope protein, seen from above, before attaching to the CD4 protein. The green shape shows how it opens and twists after attaching to CD4 (dark blue ribbon on the green shape). The image at bottom shows a side view of the HIV protein before (orange) and after (green). Credit R. Holland Cheng Usage Restrictions May be used with credit to the source. 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.