How SIV Thwarts Cellular Blockade of Virion Budding (image) University of California - Berkeley Share Print E-Mail Caption The tetherin protein (green) on the surface of monkey, ape and human cells inhibits the release of SIV virions from the cell (left). SIV overcomes this restriction by expressing the protein Nef (yellow), which down-regulates tetherin by tying it to the protein AP-2 (purple), channeling it to be destroyed. This tight Nef binding is absent in humans as a result of a mutation in tetherin. The inability of SIV to destroy human tetherin was one of the major barriers to crossover of SIV to humans. Credit UC Berkeley image by Cosmo Buffalo Usage Restrictions None Share Print E-Mail 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.