Apelin is the endogenous ligand for the G-protein-coupled APJ receptor. It is produced as a pre-proprotein of 77 amino acids. Upon signal peptide cleavage, the proprotein of 55 amino acids may generate several active fragments: Apelin-36 (36 aa corresponding to the sequence 42-77), Apelin-17 (17 aa corresponding to the sequence 61-77) and Apelin-13 (13 aa corresponding to the sequence 65-77). Apelin-13 may undergo a pyroglutamylation at its N-terminal glutamine residue.
This 36-amino-acid apelin peptide, predicted to comprise the mature form, specifically inhibits infection of APJ-expressing cells by a diverse group of primary HIV-1 viruses.