The C1q/TNF-related proteins (CTRP) form a family of secreted proteins that are structurally related to the complement factor C1q and the cytokine tumor necrosis factor-α (TNF-α). Their main role is in anti-inflammatory activation, insulin sensitization and regulation of blood lipids. The family comprises at least 15 members in humans: CTRP1 to CTRP15 (including adiponectin, also known as CTRP1. CTRP proteins are characterized by the following structural features:
- Signal peptide: For secretion into the extracellular space.
- Variable region: At the N-terminal end.
- Collagen-like domain: Enables oligomerization (e.g. trimer, hexamer formation).
- Globular C1q domain: This region is crucial for biological activity, similar to that of TNF family members.