Function and Characterization of an Alanine Dehydrogenase Homolog From Nocardia seriolae

Fish nocardiosis is a chronic, systemic, granulomatous disease in aquaculture. Nocardia seriolae has been reported to be one of the main pathogenic bacteria of fish nocardiosis. There are few studies on the associated virulence factors and pathogenesis of N. seriolae. Alanine dehydrogenase (ALD), which may be a secreted protein, was discovered by analysis using bioinformatics methods throughout the whole genomic sequence of N. seriolae. Nevertheless, the roles of ALD and its homologs in the pathogenesis of N. seriolae are not demonstrated. In this study, the function of N. seriolae ALD (NsALD) was preliminarily investigated by gene cloning, host cell subcellular localization, secreted protein identification, and cell apoptosis detection. Identification of the extracellular products of N. seriolae via mass spectrometry (MS) analysis revealed that NsALD is a secreted protein. In addition, subcellular localization of NsALD-GFP recombinant protein in fathead minnow (FHM) cells showed that the strong green fluorescence co-localized with the mitochondria. Moreover, apoptosis assays demonstrated that the overexpression of NsALD induces apoptosis in FHM cells. This study may lay the foundation for further exploration of the function of NsALD and facilitate further understanding of the pathogenic mechanism and the associated virulence factors of N. seriolae.