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.