| Proteolytic degradation by the ubiquitin (Ub) system is essential for normal cell cycle progression, cellular differentiation and stress responses. Proteins conjugated to Ub are marked for progressive degradation by the 26S proteosome. Additional members of the Ub-like protein family include the small ubiquitin-related modifier (SUMO) proteins SUMO-1, 2 and 3. Sumoylation targets proteins to a variety of cellular processing, including nuclear transport, transcriptional regulation, apoptosis and protein stability. NEDD4 is a Ub protein ligase that interacts with the epithelial Na+ channel (ENaC) to regulate cellular Na+ concentration. Two isoforms, designated NEDD4-1 and NEDD4-2 exist. NEDD8 functions as an important regulatory component of cell proliferation. |
