Spatial heterogeneity of glycogen and its metabolizing enzymes in hyphal tip cells of Aspergillus nidulans

Glycogen is a homopolymer of glucose and a ubiquitous cellular-storage carbon. This study investigated which Aspergillus nidulans genes are involved in glycogen metabolism. Gene disruptants of predicted glycogen synthase (gsyA) and glycogenin (glgA) genes accumulated less cellular glycogen than the wild-type strain, indicating that GsyA and GlgA synthesize glycogen like other eukaryotes. The gene disruption of gphA encoding glycogen phosphorylase increased the amount of glycogen more during the stationary phase that accompanies carbon-source limitation. Fluorescence-tagged GsyA and GphA were distributed in the cytosol and formed punctate and filamentous structures, respectively. Carbon starvation elongated the GphA filaments and increased their numbers. These structures were more frequently located in the basal regions of tip cells and in their adjacent cells than in the apical regions of tip cells. Cellular glycogen visualized using a fluorescent glucose analog accumulated in cytoplasmic puncta that were more prevalent in the basal regions like GsyA. The colocalization of glycogen and GsyA at punctate structures in the tip and sub-apical cells probably represents the cellular machinery for synthesizing glycogen. More frequent colocalization in the basal, than in the apical regions of the tip cells indicated that the tip cells differentiate the subcellular regions to synthesizing glycogen. Our findings of glycogen, GsyA and GphA distribution evoke the spatial heterogeneity of glycogen metabolism in fungal hyphae.