Abstract
Secretory proteins are subjected to stringent quality control systems in the endoplasmic reticulum (ER) which include the targeting of misfolded proteins for proteasomal destruction via the ER-associated degradation (ERAD) pathway. Since deletion of ERAD genes in the filamentous fungus Aspergillus niger had hardly any effect on growth, this study investigates whether autophagy might function as an alternative process to eliminate misfolded proteins from the ER. We generated A. niger double mutants by deleting genes essential for ERAD (derA) and autophagy (atg1 or atg8), and assessed their growth both under normal and ER stress conditions. Sensitivity toward ER stress was examined by treatment with dithiothreitol (DTT) and by expressing a mutant form of glucoamylase (mtGlaA::GFP) in which disulfide bond sites in GlaA were mutated. Misfolding of mtGlaA::GFP was confirmed, as mtGlaA::GFP accumulated in the ER. Expression of mtGlaA::GFP in ERAD and autophagy mutants resulted in a twofold higher accumulation in ΔderA and ΔderAΔatg1 strains compared to Δatg1 and wild type. As ΔderAΔatg1 mutants did not show increased sensitivity toward DTT, not even when mtGlaA::GFP was expressed, the results indicate that autophagy does not act as an alternative pathway in addition to ERAD for removing misfolded proteins from the ER in A. niger.