Proteins to Liposomes

The biotinylated liposomes prepared by this procedure may be stored under an inert-gas atmosphere at 4°C for long periods without degradation. 

Covalent attachment of proteins to the surface of liposomal bilayers is done through reactive sites created on the head groups of phospholipids with the intermediary use of a crosslinker or other activating agent. The lipid functional groups described in Section 1 of this chapter are modified according to the methods discussed in Section 2 to be reactive toward specific target 

Characterizing the resultant complex for the amount of protein per liposome is somewhat more difficult than in other protein conjugation applications. The protein-liposome composition is highly dependent on the size of each liposomal particle, the amount of protein charged to the reaction, and the mole quantity of reactive lipid present in the bilayer construction. An approach to solving this problem is presented by Hutchinson et al. (1989). In analyzing at least 17 different protein-liposome preparations, the ratio of proteindipid content (pg protein/pg lipid) in most of the complexes ranged from a low of about 4 to as much as 675. In some instances, however, up to 6,000 molecules of a particular protein could be incorporated into each liposome. 

The following sections present suggested protocols for creating protein-bearing liposomes. Each method utilizes specific lipid modifications to form reactive groups capable of targeting amines, sulfhydryls, aldehydes, or carboxylates on the protein molecules. 

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Derksen, J.T.P., and Scherphof, G.L. (1985) An improved method for the covalent coupling of proteins to liposomes. Biochim. Biopbys. Acta 814, 151-155. 

Radford GM, Ariatti M, Hawtrey AO. Cholesterylsuccinyl-N-hydroxy-succinimide as a cross-linking agent for the attachment of protein to liposomes. Biochem Pharmacol 1991 41 307. 

Weissig, V. Lasch, J. Klibanov, A.L. Torchilin, V.P. A new hydrophobic anchor for the attachment of proteins to liposomal membranes. EEBS Lett 1986, 202, 86-90. 

Harasym, T. O., Tardi, P., Longman, S. A., Ansell, S. M., Bally, M. B Cullis, P. R. C., and Choi, L. S. L. (1995) Poly(ethylene glycol)-modified phospholipids prevent aggregation during covalent conjugation of proteins to liposomes. Bioconj. Chem. 6, 187-194. 

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