Functional and Biochemical Characterization of Spermidine Synthase CauSpe3 from Candidozyma auris

Polyamines, putrescine, spermidine and spermine, are essential polycationic metabolites present in all eukaryotic cells, where they regulate fundamental processes including nucleic acid stabilization, translation, and stress responses. Spermidine synthase (SPDS), a member of the aminopropyltransferase (APT) family, catalyzes the transfer of an aminopropyl group from decarboxylated S-adenosylmethionine (dc-SAM) to putrescine to form spermidine. Although genomic analyses predict the presence of SPDS homologs in multiple fungal species, polyamine biosynthesis has not been experimentally characterized in the multidrug-resistant fungal pathogen Candidozyma auris. Here, we report the biochemical and functional characterization of the C. auris spermidine synthase, CauSpe3. The CauSPE3 gene complemented a Saccharomyces cerevisiae spe3Δ mutant demonstrating conserved function in vivo. Recombinant CauSpe3 was expressed in Escherichia coli, purified and analyzed using the fluorescence-based DAB-APT assay, which uses 1,2-diacetylbenzene (DAB) for polyamine detection. CauSpe3 catalyzed efficient conversion of putrescine to spermidine in the presence of dc-SAM, with K_half values of 65.5 ± 7.11 µM for putrescine and 66.9 ± 2.09 µM for dc-SAM, and V_max values of 7.1 ± 0.57 and 7.9 ± 0.12 nmol·µg^-1·min^-1, respectively. A catalytic-site mutant and heat-inactivated enzyme showed no detectable activity, and product formation was confirmed by means of thin-layer chromatography and mass spectrometry. These findings establish CauSpe3 as a functional spermidine synthase.