- Cbl‑c, also known as Cbl‑3, is the least studied member of the CBL family of single‑chain RING finger ubiquitin ligases, which also includes c‑Cbl and Cbl‑b. Although all three paralogues share a conserved structural framework, Cbl‑c has evolved a more specialised role, functioning primarily in epithelial tissues where it regulates receptor tyrosine kinase (RTK) signalling, cell adhesion and tissue homeostasis. Its restricted expression pattern and simplified domain architecture distinguish it from the other CBL proteins and shape its unique biological functions.
- Structurally, Cbl‑c contains the characteristic TKB (tyrosine kinase‑binding) domain, which enables recognition of phosphorylated tyrosine residues on activated receptors. This is followed by the RING finger domain, a zinc‑binding module that interacts with ubiquitin‑charged E2 enzymes (E2~Ub) and catalyses direct ubiquitin transfer to substrates. However, unlike c‑Cbl and Cbl‑b, Cbl‑c lacks several C‑terminal regulatory motifs, including the ubiquitin‑associated (UBA) domain and multiple proline‑rich regions. This streamlined architecture limits its interactions with adaptor proteins and signalling complexes, giving Cbl‑c a narrower but more focused functional profile.
- Functionally, Cbl‑c plays a central role in regulating EGFR signalling and other epithelial RTKs. Upon ligand stimulation, Cbl‑c binds phosphorylated receptors and catalyses their ubiquitination, promoting endocytosis and lysosomal degradation. This mechanism ensures that RTK signalling remains transient, preventing excessive proliferation and maintaining epithelial tissue integrity. Because epithelial cells rely heavily on tightly controlled RTK signalling for growth and differentiation, Cbl‑c serves as an important safeguard against aberrant activation.
- Cbl‑c also contributes to cell adhesion and cytoskeletal organisation, influencing how epithelial cells maintain structural stability and respond to environmental cues. Its ability to modulate RTK‑dependent pathways places it at the intersection of growth control, differentiation and tissue architecture. Although less versatile than c‑Cbl, Cbl‑c’s specialised functions are essential for maintaining epithelial homeostasis.
- The clinical significance of Cbl‑c is increasingly recognised. Mutations or loss of Cbl‑c have been associated with epithelial cancers, including lung, breast and colorectal carcinoma. In these contexts, impaired ubiquitination of RTKs leads to prolonged signalling, uncontrolled proliferation and enhanced metastatic potential. Some tumours exhibit reduced Cbl‑c expression, suggesting that it may function as a tumour suppressor in epithelial tissues. Understanding how Cbl‑c is dysregulated in cancer may provide new opportunities for targeted therapies aimed at restoring proper RTK turnover.
- Although Cbl‑c shares catalytic features with c‑Cbl, its biological roles are more specialised and tissue‑specific. Its simplified domain structure limits adaptor interactions but enhances its suitability for rapid, direct regulation of epithelial RTKs. This functional divergence highlights the evolutionary flexibility of the CBL family, allowing each paralogue to fulfil distinct physiological roles.
- In summary, Cbl‑c is a specialised RING finger ubiquitin ligase that maintains epithelial tissue homeostasis by regulating RTK signalling, cell adhesion and growth control. Its restricted expression pattern, streamlined architecture and focused functions distinguish it from c‑Cbl and Cbl‑b. Dysregulation of Cbl‑c contributes to epithelial cancers, making it an important subject of ongoing research in cell signalling and oncology.
