Non-natural cell surface receptors: Synthetic peptides capped with N-cholesterylglycine efficiently deliver proteins into mammalian cells

Protein toxins such as shiga toxin and cholera toxin penetrate into cells by binding small molecule-based cell surface receptors localized to cholesterol and sphingolipid-rich lipid raft subdomains of cellular plasma membranes. Molecular recognition between these toxins and their receptors triggers endocytic protein uptake through endogenous membrane trafficking pathways. We report herein the synthesis of functionally related non-natural cell surface receptors comprising peptides capped with N-cholesterylglycine as the plasma membrane anchor. The peptide moieties of these receptors were based on high-affinity epitopes of anti-hemaglutinin antibodies (anti-HA), anti-Flag antibodies, and a moderate-affinity Strep Tag II peptide ligand of the streptavidin protein from Streptomyces avidini. These non-natural receptors were directly loaded into plasma membranes of Jurkat lymphocytes to display peptides from lipid rafts on the cell surface. Molecular recognition between these receptors and added cognate anti-HA, anti-Flag, or streptavidin proteins resulted in rapid clathrin-mediated endocytosis; fluorescent target proteins were completely internalized within 4-12 h of protein addition. Analysis of protein uptake by epifluorescence microscopy and flow cytometry revealed intracellular fluorescence enhancements of 100-fold to 200-fold (10 muM non-natural receptor) with typically >99% efficiency. This method enabled intracellular delivery of a functional Escherichia coli beta-galactosidase enzyme conjugated to Protein A from Staphylococcus aureus. We termed this novel delivery strategy synthetic receptor targeting, which is an efficient method to enhance macromolecular uptake by decorating mammalian cells with chemically defined synthetic receptors that access the molecular machinery controlling the organization of cellular plasma membranes.