title: The SH3/PH domain protein AgBoi1/2 collaborates with the Rho-type GTPase AgRho3 to prevent nonpolar growth at hyphal tips of Ashbya gossypii.
abstract: Unlike most other cells, hyphae of filamentous fungi permanently elongate and lack nonpolar growth phases. We identified AgBoi1/2p in the filamentous ascomycete Ashbya gossypii as a component required to prevent nonpolar growth at hyphal tips. Strains lacking AgBoi1/2p frequently show spherical enlargement at hyphal tips with concomitant depolarization of actin patches and loss of tip-located actin cables. These enlarged tips can repolarize and resume hyphal tip extension in the previous polarity axis. AgBoi1/2p permanently localizes to hyphal tips and transiently to sites of septation. Only the tip localization is important for sustained elongation of hyphae. In a yeast two-hybrid experiment, we identified the Rho-type GTPase AgRho3p as an interactor of AgBoi1/2p. AgRho3p is also required to prevent nonpolar growth at hyphal tips, and strains deleted for both AgBOI1/2 and AgRHO3 phenocopied the respective single-deletion strains, demonstrating that AgBoi1/2p and AgRho3p function in a common pathway. Monitoring the polarisome of growing hyphae using AgSpa2p fused to the green fluorescent protein as a marker, we found that polarisome disassembly precedes the onset of nonpolar growth in strains lacking AgBoi1/2p or AgRho3p. AgRho3p locked in its GTP-bound form interacts with the Rho-binding domain of the polarisome-associated formin AgBni1p, implying that AgRho3p has the capacity to directly activate formin-driven actin cable nucleation. We conclude that AgBoi1/2p and AgRho3p support polarisome-mediated actin cable formation at hyphal tips, thereby ensuring permanent polar tip growth.
authors.affiliation: Molecular Microbiology, Biozentrum der Universität Basel, Klingelbergstrasse 50, 4056 Basel, Switzerland. peter.philippsen@unibas.ch
authors.forename: Philipp
authors.initials: P
authors.lastname: Knechtle
authors.forename: Jürgen
authors.initials: J
authors.lastname: Wendland
authors.forename: Peter
authors.initials: P
authors.lastname: Philippsen
mesh_terms: D000483:Alleles; D001203:Ascomycota / Q000166:cytology*; D016764:Cell Polarity; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism*; D025301:Hyphae / Q000166:cytology*; D008840:Microfilament Proteins / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D011485:Protein Binding; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017385:Sequence Homology; D015398:Signal Transduction; D020798:Two-Hybrid System Techniques; D020741:rho GTP-Binding Proteins / Q000378:metabolism*; D018909:src Homology Domains
chemical_list: D005656:Fungal Proteins; D008840:Microfilament Proteins; D011993:Recombinant Fusion Proteins; D020741:rho GTP-Binding Proteins