Small-molecule inhibitors for the Prp8 intein as antifungal agents

Self-splicing proteins, called inteins, are present in many human pathogens, including the emerging fungal threats Cryptococcus neoformans (Cne) and Cryptococcus gattii (Cga), the causative agents of cryptococcosis. Inhibition of protein splicing in Cryptococcus sp. interferes with activity of the only intein-containing protein, Prp8, an essential intron splicing factor. Here, we screened a small-molecule library to find addititonal, potent inhibitors of the Cne Prp8 intein using a split-GFP splicing assay. This revealed the compound 6G-318S, with IC50 values in the low micromolar range in the split-GFP assay and in a complementary split-luciferase system. A fluoride derivative of the compound 6G-318S displayed improved cytotoxicity in human lung carcinoma cells, although there was a slight reduction in the inhibition of splicing. 6G-318S and its derivative inhibited splicing of the Cne Prp8 intein in vivo in Escherichia coli and in C. neoformans. Moreover, the compounds repressed growth of WT C. neoformans and C. gattii. In contrast, the inhibitors were less potent at inhibiting growth of the intein-less Candida albicans. Drug resistance was observed when the Prp8 intein was overexpressed in C. neoformans, indicating specificity of this molecule toward the target. No off-target activity was observed, such as inhibition of serine/cysteine proteases. The inhibitors bound covalently to the Prp8 intein and binding was reduced when the active-site residue Cys1 was mutated. 6G-318S showed a synergistic effect with amphotericin B and additive to indifferent effects with a few other clinically used antimycotics. Overall, the identification of these small-molecule intein-splicing inhibitors opens up prospects for a new class of antifungals.

Automated summary

A small-molecule library was screened to find potent inhibitors of the Cne Prp8 intein, leading to the identification of compound 6G-318S with potential antifungal properties. These inhibitors showed specific action against Cryptococcus species and demonstrated synergistic effects with certain antimycotics. Their binding to the target Prp8 intein was covalent and mutation of the active-site residue reduced binding, highlighting their potential as a new class of antifungal drugs.