The small GTPase ArfA controls secretion, morphology, and growth in Aspergillus niger via actin ring positioning

In filamentous fungi, growth and protein secretion occurs predominantly at the hyphal tip. This requires coordinated regulation of multiple processes, including vesicle trafficking, exocytosis, and endocytosis, which are facilitated by a complex cytoskeletal apparatus. In this study, functional analyses of the small GTPase ArfA from Aspergillus niger demonstrate that this protein functionally complements the Saccharomyces cerevisiae ARF1/2, and that this protein is essential for A. niger, where it regulates hyphal growth rates, protein secretion, and hyphal morphology. ArfA co-localizes to Golgi equivalents and post-Golgi carriers, but not the endoplasmic reticulum in hyphae. Moreover, localization of the endocytic machinery, visualized via fluorescent tagging of the actin ring, was found to be abnormal in ArfA under- and overexpressed conditions, indicating that ArfA mechanistically regulates secretion by affecting actin ring positioning. Finally, we provide evidence that secretion in A. niger occurs subapically, which may be a mechanism to compensate for defective and/or excess secretion at the hyphal apex due to ArfA misregulation. Taken together, our results demonstrate that ArfA fulfils multiple functions in the secretory pathway of A. niger. We propose that ArfA is a critical regulator that controls the endocytotic machinery at the hyphal apex.