# Covalent modification of a glutamic acid inspired by HaloTag technology

## TL;DR

A new method inspired by HaloTag technology enables covalent modification of glutamic acid in proteins, offering a way to target previously hard-to-reach amino acids.

## Contribution

A novel technique for covalent modification of glutamates using alkyl bromide warheads in non-covalent inhibitors.

## Key findings

- DeltaTag covalently labels PDEδ at p.E88 under biologically relevant conditions.
- DeltaTag disrupts the PDEδ-Rheb-mTORC1 axis and inhibits cancer cell proliferation.
- The method shows promise for modifying proteins with lipophilic binding sites lacking reactive amino acids.

## Abstract

For targeted covalent protein modification at low-reactivity aspartates and glutamates, new methods are in high demand. We report a technique inspired by the HaloTag technology, which employs nucleophilic substitution at chloroalkane-functionalised ligands by a specific aspartate residue. Embedding of alkyl bromide warheads into non-covalent inhibitors enables covalent modification of a glutamate in the lipoprotein binding chaperone - phosphodiesterase of retinal rod subunit delta (PDEδ), which shuttles prenylated lipoproteins between cellular membranes and thereby mediates their activity. Its hydrophobic ligand-binding pocket contains p.E88 as the only accessible nucleophile for covalent targeting. We show that a covalent inhibitor, termed DeltaTag, overcomes limitations of non-covalent inhibitors. DeltaTag labels PDEδ at its p.E88 under biologically relevant conditions, modulates mammalian target of rapamycin (mTOR) signalling by disrupting the PDEδ-Rheb (Ras homologue enriched in brain)-mTORC1 (mTOR complex 1) axis and inhibits cancer cell proliferation. This proof-of-concept study demonstrates that the design strategy holds promise for the covalent modification of proteins with lipophilic binding sites that lack accessible reactive amino acids but contain specific carboxylates.

New methods for targeted covalent protein modification at low reactivity aspartates and glutamates are of high interest. Here, the authors report a technique inspired by the HaloTag technology, which employs a covalent conjugation reaction between ligands with a reactive chloroalkane linker and a specific aspartic acid, and use it to covalently modify lipoprotein chaperone PDEδ at a binding site glutamic acid.