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Eukaryotic Cell, January 2008, p. 141-153, Vol. 7, No. 1
1535-9778/08/$08.00+0     doi:10.1128/EC.00346-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Ras GTPase-Activating Protein Regulation of Actin Cytoskeleton and Hyphal Polarity in Aspergillus nidulans{triangledown} ,{dagger}

Laura Harispe,1,2,4 Cecilia Portela,1 Claudio Scazzocchio,2,3 Miguel A. Peñalva,4* and Lisette Gorfinkiel1,#

Departamento de Bioquímica, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo CP11400, Uruguay,1 Institut de Génétique et Microbiologie, Université de Paris-Sud, Centre Universitaire d'Orsay, 91405 Orsay, France,2 Department of Microbiology, Imperial College London, Flowers Building, Armstrong Road, London SW7 2AZ, United Kingdom,3 Departamento de Microbiología Molecular, Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, Madrid 28040, Spain4

Received 20 September 2007/ Accepted 8 November 2007

Aspergillus nidulans gapA1, a mutation leading to compact, fluffy colonies and delayed polarity establishment, maps to a gene encoding a Ras GTPase-activating protein. Domain organization and phylogenetic analyses strongly indicate that GapA regulates one or more "true" Ras proteins. A gapA{Delta} strain is viable. gapA colonies are more compact than gapA1 colonies and show reduced conidiation. gapA{Delta} strains have abnormal conidiophores, characterized by the absence of one of the two layers of sterigmata seen in the wild type. gapA transcript levels are very low in conidia but increase during germination and reach their maximum at a time coincident with germ tube emergence. Elevated levels persist in hyphae. In germinating conidiospores, gapA{Delta} disrupts the normal coupling of isotropic growth, polarity establishment, and mitosis, resulting in a highly heterogeneous cell population, including malformed germlings and a class of giant cells with no germ tubes and a multitude of nuclei. Unlike wild-type conidia, gapA{Delta} conidia germinate without a carbon source. Giant multinucleated spores and carbon source-independent germination have been reported in strains carrying a rasA dominant active allele, indicating that GapA downregulates RasA. gapA{Delta} cells show a polarity maintenance defect characterized by apical swelling and subapical branching. The strongly polarized wild-type F-actin distribution is lost in gapA{Delta} cells. As GapA-green fluorescent protein shows cortical localization with strong predominance at the hyphal tips, we propose that GapA-mediated downregulation of Ras signaling at the plasma membrane of these tips is involved in the polarization of the actin cytoskeleton that is required for hyphal growth and, possibly, for asexual morphogenesis.

* Corresponding author. Mailing address: Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, Madrid 28040, Spain. Phone: 34918373112, ext. 4358. Fax: 34915360432. E-mail: penalva{at}cib.csic.es

{triangledown} Published ahead of print on 26 November 2007.

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

# Deceased 9 October 2003.

Eukaryotic Cell, January 2008, p. 141-153, Vol. 7, No. 1
1535-9778/08/$08.00+0     doi:10.1128/EC.00346-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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