A New Member of the TBC1D15 Family from Chiloscyllium plagiosum: Rab GTPase-Activating Protein Based on Rab7 as a Substrate

APSL (active peptide from shark liver) is a hepatic stimulator cytokine from the liver of Chiloscyllium. It can effectively protect islet cells and improve complications in mice with alloxan-induced diabetes. Here, we demonstrate that the APSL sequence is present in the N-terminus of novel TBC (Tre-2, Bub2 and Cdc16) domain family, member 15 (TBC1D15) from Chiloscyllium plagiosum. This shark TBC1D15 gene, which contains an ORF of 2088 bp, was identified from a cDNA library of regenerating shark liver. Bioinformatic analysis showed that the gene is highly homologous to TBC1D15 genes from other species. Moreover, the N-terminus of shark TBC1D15 contains a motif of unknown function (DUF3548), which encompasses the APSL fragment. Rab-GAP activity analysis showed that shark TBC1D15 is a new member of the TBC1D15 family. These results demonstrated that shark TBC1D15 possesses Rab-GAP activity using Rab7 as a substrate, which is a common property of the TBC1D15 family. The involvement of APSL at the N-terminus of TBC1D15 also demonstrates that this protein might be involved in insulin signaling and may be associated with the development of type 2 diabetes. The current findings pave the way for further functional and clinical studies of these proteins from marine sources.