Isolation of Mutations That Bypass the Requirement of the Septation Initiation Network for Septum Formation and Conidiation inAspergillus nidulans

Abstract

The kinase cascade of the septation initiation network (SIN), first revealed in fission yeast, activates the contraction of the actomyosin ring, and plays an essential role in fungal septation. Mob1p, an evolutionarily conserved SIN protein, is associated with the most downstream kinase of this cascade in fission yeast. In this study, the mobA gene encoding a homologous protein was isolated from the filamentous fungus Aspergillus nidulans, whose mycelium is made of multinucleate cells. The MOBA protein was required for septation and conidiation, but was not essential for hyphal extension and colony formation. To identify genes that act antagonistically against the SIN, UV mutagenesis was carried out to isolate suppressor (smo) mutations that restored conidiation when MOBA was not expressed. Microscopic examination indicated that the restored conidiation was concomitant with restored septation in the absence of the MOBA protein. Eight recessive smo mutations in five complementation groups also bypassed the requirement of the SIN kinases SEPH and SIDB for septum formation and conidiation. However, none of these smo mutations affected the localization of MOBA. Among smo mutations, smoA and smoB mutations caused reduced hyphal growth and colony formation. They also rendered hypersensitivity to low doses of the microtubule-depolymerizing agent benomyl for conidiation. Therefore, in A. nidulans, proteins encoded by the smo genes likely have an antagonistic interaction against the SIN pathway to regulate septation and conidiation.

Colletotrichum orbiculare Regulates Cell Cycle G1/S Progression via a Two-Component GAP and a GTPase to Establish Plant Infection

“…By contrast, in S. pombe and A. nidulans, homologs of BUB2 and BFA1 act as negative regulators of the SIN, which is required for the onset of septum formation. SIN signaling activity is tightly regulated to ensure proper coordination of mitosis and cytokinesis (Furge et al, 1998;Li et al, 2000;Kim et al, 2006). In contrast to the results obtained so far for model yeast and fungi, we showed here that C. orbiculare homologs of BUB2 and BFA1, Co BUB2 and Co BFA1, play a major role in phase progression from G1 to S. This novel finding is supported by the following results.…”

Regulation of Septum Formation by the Bud3–Rho4 GTPase Module inAspergillus nidulans

“…A potential pathway that might link nuclear division to AnBud3 localization is the SIN, which is required for assembly of the CAR in A. nidulans (Bruno et al 2001;Kim et al 2006) and functions upstream of SepA (Sharpless and Harris 2002). To test this notion, we transformed our GFPTbud3 construct into a strain possessing the temperature-sensitive sepH 1 mutation.…”

“…Our observations that nuclear division and a functional SIN pathway are required for AnBud3 localization at septation sites are consistent with this idea. Future efforts will focus on determining whether the protein kinase constituents of the SIN (SepH, SepL, and SidB) (Bruno et al 2001;Kim et al 2006Kim et al , 2009 interact directly with AnBud3 to control its localization and/or activity. Notably, in S. pombe, Orb6, which like SidB is a member of the NDR kinase family, spatially regulates polarized growth by restricting the localization of the Cdc42 GEF Gef1 (Das et al 2009).…”

“…The SIN is a cascade of three protein kinases that is activated by a small GTPase (Krapp and Simanis 2008). In A. nidulans, the component kinases of the SIN are arranged in the pathway SepH/SepL/SidB, with SepM amd MobA serving as cofactors that regulate SepL and SidB, respectively (Kim et al 2006(Kim et al , 2009. Although SIN components localize to the spindle pole bodies, this does not appear to be a prerequisite for their subsequent recruitment to the septation site (Kim et al 2009).…”

“…Although SIN components localize to the spindle pole bodies, this does not appear to be a prerequisite for their subsequent recruitment to the septation site (Kim et al 2009). Functional analysis of SepH, ModA, and SidB demonstrate that the SIN is required for assembly of the CAR (Bruno et al 2001;Kim et al 2006). Nevertheless, the upstream activators of the SIN and its downstream effectors remain unknown.…”