Subcellular Localization of Nicotinic Acid Utilization Pathway Proteins in Aspergillus nidulans and a Putative HxnX-Dependent Piggyback Import of HxnW

A recently discovered eukaryotic nicotinic acid catabolic pathway involves the concerted action of eight Hxn enzymes including HxnX, a peroxisomal enzyme carrying a canonical PTS1 motif (SRL) at its C-terminus. None of the seven remaining pathway enzymes contains an obvious peroxisomal targeting signal (PTS). However, the nicotinic acid transporters HxnP and HxnZ, and the 2,5-dihydroxypyridine hydroxylase HxnV, carry non-canonical PTS1 motifs originally described in plants. N-terminal Gfp fusions localized HxnP and HxnZ to the plasma membrane and septa, whereas Gfp-HxnV showed diffuse cytoplasmic localization, with no detectable peroxisomal targeting. The hxnX and hxnW genes, which encode HxnX and the 5,6-dihydroxypiperidine-2-one ketoreductase HxnW, respectively, are divergently transcribed from a shared intergenic region, suggesting coordinated regulation of their expression and raising the possibility of a functional association between the two proteins. Consistent with this possibility, bimolecular fluorescence complementation (BiFC) provided evidence for an interaction between HxnX and HxnW. However, neither the BiFC fusion proteins nor the Gfp-HxnW fusion complemented the nicotinic acid utilization defects of the hxnXΔ hxnWΔ and hxnWΔ strains, respectively, and neither showed peroxisomal localization. These findings indicate that fusion of Gfp or split-Yfp fragments interferes with HxnW function, potentially by affecting protein folding, catalytic activity, protein–protein interaction, or intracellular targeting. Thus, while the observed HxnX–HxnW interaction is consistent with a possible piggyback-mediated import mechanism, the peroxisomal import of HxnW remains to be demonstrated experimentally.