Fig. 1

Types of cell fusion and the process of cell membrane fusion. a Three types of syncytia: Cells that undergo fusion are collectively referred to as syncytia and include three types—homokaryon, heterokaryon, and synkaryon. Homokaryon formation: Fusion of cells with the same type of nuclei results in a multinucleated cell. Heterokaryon formation: Fusion of cells with distinct types of nuclei forms a multinucleated cell. Synkaryon formation: A single-nucleus cell is formed through the fusion of cells with either the same or different types of nuclei. b Membrane fusion process: Cells approach and interact with each other (outer coat interaction). This interaction leads to the formation of a stalk-like structure. The stalk then expands into a diaphragm. Finally, the fusion pore forms, completing the membrane fusion. c Syncytin-1 mediated fusion in trophoblasts: The receptor-binding domain (RBD) of syncytin-1 binds to the Na-dependent neutral amino acid transporter type 2 (ASCT2). The surface unit domain (SU) is removed from the transmembrane domain (TM) by cleaving disulfide (SS) bonds. The fusion peptide (FP) is inserted into the target plasma membrane, initiating host plasma membrane bending