The E2 enzyme, also known as a ubiquitin-conjugating enzyme, plays a crucial role in the process of protein degradation within cells. It is responsible for transferring ubiquitin, a small protein, to target proteins that need to be marked for degradation. This tagging process is essential for maintaining cellular health and regulating various biological functions. E2 enzymes work in conjunction with E1 enzymes, which activate ubiquitin, and E3 enzymes, which facilitate the transfer of ubiquitin to the target protein. This three-step process is part of the ubiquitin-proteasome system, a key mechanism for controlling protein levels and removing damaged or misfolded proteins from the cell.