The Core Protein of the Syndecan-4 Ectodomain on Small Extracellular Vesicles Promotes Fusion with Endosomes

Small extracellular vesicles (sEVs) can efficiently transfer payloads, such as nucleic acids, into the cytosol of cells. Endocytosis is a major pathway for the intracellular trafficking of sEVs, and some sEVs can fuse with late endosomes to release cargoes into the cytosol. Our understanding of the mechanisms regulating sEV composition heterogeneity and the molecular mechanism of the membrane fusion between sEVs and endosomes is limited. Here, we show that the core protein of the syndecan-4 ectodomain, the syndecan-4 ectodomain without heparan sulfate, on sEVs promoted this membrane fusion. In an <i>in vitro</i> lipid-mixing assay, the core protein of the syndecan-4 ectodomain was found to promote membrane fusion under the acidic conditions that are found in late endosomes. A recombinant core protein of the syndecan-4 ectodomain showed higher fusion activity than the domain with heparan sulfate. Conformational changes in the core protein were observed depending on the pH. The involvement of the core protein in membrane fusion was further investigated in a cell-based assay. Our results indicated that the conformation of the core protein was changed in late endosomes, which induced membrane fusion along with an increase in the membrane fluidity of the sEVs. This mechanism involving fusion proteins is similar to the mechanism of many virus infection systems. Syndecan-4, which is initially a glycoprotein containing heparan sulfate on the plasma membrane, has various functions, including cell adhesion and cell signaling, which are mainly thought to be facilitated through the heparan sulfate, and our results indicate the versatility of syndecan-4 actions <i>in vivo</i>.