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Eukaryotic Cell, August 2008, p. 1278-1288, Vol. 7, No. 8
1535-9778/08/$08.00+0     doi:10.1128/EC.00039-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Aspergillus nidulans ArfB Plays a Role in Endocytosis and Polarized Growth {triangledown} ,{dagger}

Soo Chan Lee,1,{ddagger} Sabrina N. Schmidtke,2 Lawrence J. Dangott,2 and Brian D. Shaw1*

Program for the Biology of Filamentous Fungi, Department of Plant Pathology and Microbiology, Texas A&M University, 2132 TAMU, College Station, Texas 77843,1 Department of Biochemistry and Biophysics, Texas A&M University, 2128 TAMU, College Station, Texas 778432

Received 1 February 2008/ Accepted 29 May 2008

Filamentous fungi undergo polarized growth throughout most of their life cycles. The Spitzenkörper is an apical organelle composed primarily of vesicles that is unique to filamentous fungi and is likely to act as a vesicle supply center for tip growth. Vesicle assembly and trafficking are therefore important for hyphal growth. ADP ribosylation factors (Arfs), a group of small GTPase proteins, play an important role in nucleating vesicle assembly. Little is known about the role of Arfs in filamentous hyphal growth. We found that Aspergillus nidulans is predicted to encode six Arf family proteins. Analysis of protein sequence alignments suggests that A. nidulans ArfB shares similarity with ARF6 of Homo sapiens and Arf3p of Saccharomyces cerevisiae. An arfB null allele (arfB disrupted by a transposon [arfB::Tn]) was characterized by extended isotropic growth of germinating conidia followed by cell lysis or multiple, random germ tube emergence, consistent with a failure to establish polarity. The mutant germ tubes and hyphae that do form initially meander abnormally off of the axis of polarity and frequently exhibit dichotomous branching at cell apices, consistent with a defect in polarity maintenance. FM4-64 staining of the arfB::Tn strain revealed that another phenotypic characteristic seen for arfB::Tn is a reduction and delay in endocytosis. ArfB is myristoylated at its N terminus. Green fluorescent protein-tagged ArfB (ArfB::GFP) localizes to the plasma membrane and endomembranes and mutation (ArfBG2A::GFP) of the N-terminal myristoylation motif disperses the protein to the cytoplasm rather than to the membranes. These results demonstrate that ArfB functions in endocytosis to play important roles in polarity establishment during isotropic growth and polarity maintenance during hyphal extension.

* Corresponding author. Mailing address: Program for the Biology of Filamentous Fungi, Department of Plant Pathology and Microbiology, Texas A&M University, 2132 TAMU, College Station, TX 77843. Phone: (979) 862-7518. Fax: (979) 845-6483. E-mail: bdshaw{at}tamu.edu

{triangledown} Published ahead of print on 6 June 2008.

{dagger} Supplemental material for this article may be found at http://ec.asm.org/.

{ddagger} Present address: Department of Molecular Genetics and Microbiology, Duke University Medical Center, 320 Carl Building, Box 3546, Durham, NC 27710.

Eukaryotic Cell, August 2008, p. 1278-1288, Vol. 7, No. 8
1535-9778/08/$08.00+0     doi:10.1128/EC.00039-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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