Protein-liposome conjugates using cysteine-lipids and native chemical ligation

Liposomes have become popular drug delivery vehicles and have more recently also been applied as contrast agents for mol. imaging. Most current methods for functionalization of liposomes with targeting proteins rely on reactions of amine or thiol groups at the protein exterior, which generally result in nonspecific conjugation at multiple sites on the protein. In this study, we present native chem. ligation (NCL) as a general method to covalently couple recombinant proteins in a highly specific and chemoselective way to liposomes contg. cysteine-functionalized phospholipids. A cysteine-functionalized phospholipid (Cys-PEG-DSPE) was prepd. and shown to readily react with the MESNA thioester of EYFP, which was used as a model protein. Characterization of the EYFP-liposomes using fluorescence spectroscopy showed full retention of the fluorescent properties of conjugated EYFP and provides a lower limit of 120 proteins per liposome. The general applicability of NCL was further tested using CNA35, a collagen-binding protein recently applied in fluorescent imaging of collagen. NCL of CNA35 thioester yielded liposomes contg. .apprx.100 copies of CNA35 per liposome. The CNA35-liposomes were shown to be fully functional and bind collagen with a 150-fold higher affinity compared to CNA35. Our results show that NCL is an attractive addn. to existing conjugation methods that allows direct, covalent, and highly specific coupling of recombinant proteins to liposomes and other lipid-based assemblies.