Covalent Modification of Glutamic Acid Inspired by HaloTag Technology

For targeted covalent protein modification at low reactivity aspartates and glutamates, new methods are in high demand. Inspired by the HaloTag technology we have developed a new technique which employs a reaction between chloroalkane-functionalised ligands and a specific glutamate residue. The lipoprotein chaperone PDEδ shuttles prenylated lipoproteins between cellular membranes and, thereby, mediates their activity. In cells, reversible PDEδ inhibition is efficiently counterbalanced by Arl2/3-mediated inhibitor release calling for covalent inhibitor development. However, the hydrophobic ligand binding site contains only Glu88 as accessible nucleophile. Inspired by the HaloTag technology, we have developed a novel covalent PDEδ inhibitor chemotype with alkyl bromide warheads which targets glutamate E88. The best covalent inhibitor, termed DeltaTag, overcomes Arl2-mediated release, modulates signal transduction through the mTOR pathway and inhibits cancer cell proliferation. The design strategy promises to be applicable also to other proteins with carboxylate residues embedded in hydrophobic binding sites, such as other lipoprotein chaperones.