N-Terminus Plays a Critical Role for Stabilizing the Filamentous Assembly and the Antifungal Activity of Bg_9562.

Bg_9562, a prophage tail-like protein was earlier shown to be required for bacterial mycophagy by Burkholderia gladioli strain NGJ1. The purified protein exhibited broad-spectrum antifungal activity; however, the structural and mechanistic details vis-à-vis its activity remained elusive. In this study, we have structurally characterized the protein Bg_9562 using negatively stained transmission electron microscopy, molecular modeling and mutagenesis. We find that Bg_9562 shows structural similarity to Gp13, a tail assembly chaperone. The transmission electron microscopy revealed that, Bg_9562 forms long flexible tubular structures. Molecular modeling of the filament like structure divulges that the inter subunit contacts are meditated largely through hydrophobic interactions. Using mutagenesis, we demonstrate that the N-terminal residues of the protein when deleted results in reduced activity and destabilization of filament formation. Overall, structure-function analysis opens up avenues for further utilization of the protein as a potent antifungal molecule. IMPORTANCE Burkholderia gladioli strain NGJ1, isolated from healthy rice seedling, was earlier demonstrated to have mycophagous properties on a broad range of fungi, including Rhizoctonia solani, a causal agent of deadly sheath blight disease of rice. The purified Bg_9562 protein exerts broad-spectrum antifungal activity. The protein also inhibits the growth of laboratory strain of Candida, an opportunistic human pathogen. In this study, we structurally characterize Bg_9562 using a combination of negative staining transmission electron microscopy, molecular modeling, mutagenesis, and functional antifungal assay. We show that the protein assembles into long filament like structures stabilized by N-terminus residues and this region is important for its activity. Our study has implications in utilizing Bg_9562 or its derivatives as antifungal molecule(s) which will provide environmentally friendly control of fungal diseases of plants and animals.