The present application relates to methods of producing exosomes. The application also provides a method for preparing a protein composition comprising culturing an exosome-producing cell expressing a Nef-fusion protein comprising a Nef-derived peptide fused to a protein of interest; isolating exosomes from the exosome-producing cell culture; and purifying the protein of interest from the isolated exosomes. The application further discloses compositions that comprise exosomes containing the Nef-fusion protein, as well as methods of using the Nef-fusion protein and exosomes containing the Nef-fusion protein.