25. E1, E2, ubiquitin, ATP, and both native and heat-inactivated (denoted by an asterisk, *) GST-ARD1 (1-574), GST, and GST-cytohesin-1 (C-1) were mixed and run on a gel. A western blot was performed with an anti-ubiquitin antibody. a. Based on these results, what three proteins are required for ubiquitination