The invention relates to N protein-protein of interest fusion proteins, optionally in the form of soluble N protein-protein of interest/P protein complexes, the N and P proteins being proteins of a virus of the Paramyxoviridae family. When the protein of interest is an antigen, the invention relates also to vaccinal compositions and diagnostic reagents comprising those N protein-antigen fusion proteins or those N protein-antigen/P protein complexes. The N protein-protein of interest fusion protein can also be used as a "vector" for transporting into cells therapeutic molecules of interest, such as antivirals or anticancer agents.