Figure 2.

Potential models of poly-ubiquitination. Sequential addition of ubiquitin refers to the process of ubiquitin transferring from the E1, to the E2 and then E3 ligase onto the substrate, this process would then be repeated for each additional ubiquitin added to the chain. However this model fails to describe how the E3-ligase which interacts with the substrate protein can physically reach the end of the ubiquitin chain. Some protein ubiquitination is mediated by multiple E3-ligases acting with the E3-ligase. The first E2-ligase mediates mono-ubiquitination, and the second E2-ligase promotes poly-ubiquitination. Some E2-ligases can assist in the transfer of pre-synthesized poly-ubiquitin onto the substrate. Finally, some proteins have E4 ligase activity. When added to the reaction, E4 ligases promote poly-ubiquitin chain extension.