Timely septation requires SNAD-dependent spindle pole body localization of the septation initiation network components in the filamentous fungus Aspergillus nidulans.

In the filamentous fungus Aspergillus nidulans, cytokinesis/septation is triggered by the septation initiation network (SIN), which first appears at the spindle pole body (SPB) during mitosis. The coiled-coil protein SNAD is associated with the SPB and is required for timely septation and conidiation. We have determined that SNAD acted as a scaffold protein that is required for the localization of the SIN proteins of SIDB and MOBA to the SPB. Another scaffold protein SEPK, whose localization at the SPB was dependent on SNAD, was also required for SIDB and MOBA localization to the SPB. In the absence of either SEPK or SNAD, SIDB/MOBA successfully localized to the septation site, indicating that their earlier localization at SPB was not essential for their later appearance at the division site. Unlike their functional counterparts in fission yeast, SEPK and SNAD were not required for vegetative growth but only for timely septation. Furthermore, down-regulation of negative regulators of the SIN suppressed the septation and conidiation phenotypes due to the loss of SNAD. Therefore, we conclude that SPB localization of SIN components is not essential for septation per se, but critical for septation to take place in a timely manner in A. nidulans.

Isolation of mutations that bypass the requirement of the septation initiation network for septum formation and conidiation in Aspergillus nidulans.

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.