Inteins as Drug Targets and Therapeutic Tools

Abstract

Multidrug-resistant pathogens are of significant concern in recent years. Hence new antifungal and anti-bacterial drug targets are urgently needed before the situation goes beyond control. Inteins are polypeptides that self-splice from exteins without the need for cofactors or external energy, resulting in joining of extein fragments. Inteins are present in many organisms, including human pathogens such as Mycobacterium tuberculosis, Cryptococcus neoformans, C. gattii, and Aspergillus fumigatus. Because intein elements are not present in human genes, they are attractive drug targets to develop antifungals and antibiotics. Thus far, a few inhibitors of intein splicing have been reported. Metal-ions such as Zn²⁺ and Cu²⁺, and platinum-containing compound cisplatin inhibit intein splicing in M. tuberculosis and C. neoformans by binding to the active site cysteines. A small-molecule inhibitor 6G-318S and its derivative 6G-319S are found to inhibit intein splicing in C. neoformans and C. gattii with a MIC in nanomolar concentrations. Inteins have also been used in many other applications. Intein can be used in activating a protein inside a cell using small molecules. Moreover, split intein can be used to deliver large genes in experimental gene therapy and to kill selected species in a mixed population of microbes by taking advantage of the toxin-antitoxin system. Furthermore, split inteins are used in synthesizing cyclic peptides and in developing cell culture model to study infectious viruses including SARS-CoV-2 in the biosafety level (BSL) 2 facility. This mini-review discusses the recent research developments of inteins in drug discovery and therapeutic research.

Keywords: anti-microbial; drug target; inhibitor; intein; therapeutic tool.

Graphical Abstract

FIGURE 1

Structures of inteins, intein-ligand complexes, and small molecule inhibitors of intein splicing and hedgehog cholesterolysis. (A) The Prp8 intein of C. neoformans and Zn²⁺ (Green et al., 2019). (B) The DnaBi1 intein of M. smegmatis and Zn²⁺ (Woods et al., 2020). (C), The RecA intein of M. tuberculosis in complex with platinum and TCEP (Chan et al., 2016). (D) The Prp8 intein of C. gattii and cisplatin (Li et al., 2019). (E) The C-terminal 17 kDa fragment of drosophila hedgehog showing catalytic residues in cholesterolysis (Hall et al., 1997). (F) Structures of small molecules that inhibit intein splicing and hedgehog cholesterolysis.

FIGURE 2

Use of split intein in therapy and pathogenesis studies. (A) Large gene delivery in gene therapy (Tornabene et al., 2019; Lim et al., 2020). (B) Intein mediated toxin reconstitution inside the cells for selective cell killing in tumor therapy (Purde et al., 2020). (C) Development of a BSL2 cell culture model for SARS-COV-2 in CaCo2 cells (Ju et al., 2021).