Tracking exocytosis of a GPI-anchored protein in Aspergillus nidulans

Secretion of the glycosylphosphatidylinositol-anchored protein (GPI-AP) EglC was investigated in the filamentous fungus Aspergillus nidulans, exploiting a sucrose-inducible promoter to conditionally express the protein in cells blocked at different steps of exocytosis. EglC is delivered to the cell surface in a polarized fashion, but appears to redistribute rapidly toward apico-distal regions. Inactivation of SarA^Sar1 mediating COPII vesicle biogenesis resulted in the accumulation of EglC in the endoplasmic reticulum (ER) but, rather than concentrating in ER-exit-sites, the reporter labeled the ER uniformly. Abnormal posttranslational modifications of EglC were detected in sarAts and sed5ts mutants, suggesting that blocking COPII biogenesis or traffic in the ER/Golgi interface might affect GPI remodeling. EglC delivery to the plasma membrane requires, besides Golgi function, the TRAPPII complex mediating the biogenesis of RAB11 secretory vesicles at the TGN, but is unaffected by the absence of RAB5, the key regulator of early endosome biogenesis/maturation. Thus, unlike the soluble extracellular enzyme inulinase, EglC is directly delivered from the TGN to the plasma membrane without involvement of endosomes. We conclude that in A. nidulans, GPI-APs follow a direct secretory pathway from the ER to the plasma membrane.