E1 Ubiquitin Activating Enzyme is a crucial protein in the ubiquitin-proteasome system, which regulates protein degradation and cellular processes. It activates ubiquitin, a small protein that tags other proteins for degradation, by forming a high-energy thioester bond with it. This activation is the first step in a cascade that ultimately leads to the attachment of ubiquitin to target proteins. Once activated, the E1 Ubiquitin Activating Enzyme transfers ubiquitin to E2 Ubiquitin Conjugating Enzymes, which then carry the ubiquitin to E3 Ubiquitin Ligases. These ligases facilitate the transfer of ubiquitin to specific substrate proteins, marking them for degradation by the proteasome, thus maintaining cellular homeostasis.