Peptides and antibodies represent two leading modalities for targeting protein receptors expressed on the surface of mammalian cells. Endogenous peptides often serve as leads for developing tools and therapeutic candidates for probing receptor function, but they often suffer from insufficient selectivity, especially when targeting members of large receptor families such as G protein-coupled receptors, GPCRs. Alternatively, antibodies are prized in biomedical research for their high specificity but typically lack any biological effect, inhibition or activation, when targeting receptors with orthosteric sites embedded within transmembrane domains.

To leverage and expand the favorable properties of peptides and antibodies we have developed methods to create conjugates comprised of peptidic GPCR ligands and antibody fragments, nanobodies. Synthesis is performed through a combination of solid-phase peptide synthesis, site-specific labeling of recombinantly expressed nanobodies, and click chemistry. These conjugates show properties superior to those of antibodies or peptides alone, including receptor specificity and in vivo biological activity.

We have further elaborated on this approach by developing chemistry that facilitates the covalent linkage between a nanobody and its binding partner through templated amide bond formation. This approach showed that covalent linkage of an agonist to its receptor resulted in enduring signaling. These tools should prove useful for mechanistic investigations of peptide-binding receptors and provide a new class of molecules with useful properties for biomedical investigation.