Regio- and Chemoselective Covalent Immobilization of Proteins through Unnatural Amino Acids

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Abstract
A general approach was developed for the regio- and chemoselective covalent immobilization of soluble proteins on glass surfaces through an unnatural amino acid created by post-translationally modifying the cysteine residue in a CaaX recognition motif with functional groups suitable for “click” chemistry or a Staudinger ligation. Farnesyl diphosphate analogues bearing ω-azide or ω-alkyne moieties were attached to the cysteine residue in Cys-Val-Ile-Ala motifs at the C-termini of engineered versions of green fluorescent protein (GFP) and glutathione S-transferase (GST) by protein farnesyltransferase. The derivatized proteins were attached to glass slides bearing linkers containing azide (“click” chemistry) or phosphine (Staudinger ligation) groups. “Click”-immobilized proteins were detected by fluorescently labeled antibodies and remained attached to the slide through two cycles of stripping under stringent conditions at 80 °C. GFP immobilized by a Staudinger ligation was detected by directly imagining the GFP fluorophore over a period of 6 days. These methods for covalent immobilization of proteins should be generally applicable. CaaX recognition motifs can easily be appended to the C-terminus of a cloned protein by a simple modification of the corresponding gene, and virtually any soluble protein or peptide bearing a CaaX motif is a substrate for protein farnesyltransferase.