Microtubule-organizing centers of Aspergillus nidulans are anchored at septa by a disordered protein

Area of Science:

• Cell Biology
• Mycology
• Cytoskeleton Dynamics

Background:

• Microtubule-organizing centers (MTOCs) are essential protein complexes regulating microtubule organization.
• In fungi like Aspergillus nidulans, septum-associated MTOCs (sMTOCs) are permanent structures, distinct from transient ones in other species.
• The molecular composition and targeting mechanisms of A. nidulans sMTOCs remain largely unknown.

Purpose of the Study:

• To investigate the protein composition and recruitment of sMTOCs in Aspergillus nidulans.
• To identify proteins responsible for targeting MTOCs to septal structures.
• To understand the role of specific proteins in sMTOC assembly and function.

Main Methods:

• Immunofluorescence microscopy to localize proteins at sMTOCs and SPBs.
• Genetic analysis using knockout strains to assess protein function in MTOC recruitment and microtubule nucleation.
• Biochemical assays to confirm protein interactions and localization.

Main Results:

• Several outer plaque proteins of spindle pole bodies (SPBs) were found at sMTOCs, but key SPB proteins like SfiA and PcpA were absent.
• The disordered protein Spa18 (Mto2) and centrosomin-domain protein ApsB (Mto1) are crucial for recruiting the γ-TuRC component GcpC to sMTOCs and initiating microtubule formation.
• The novel septum-specific protein Spa10 was identified as a key component, forming a septal pore disc and recruiting Spa18 and ApsB to the septum, which is essential for sMTOC activity.

Conclusions:

• Spa10 acts as a specific anchor for sMTOCs at the septum in Aspergillus nidulans.
• This study reveals a novel mechanism for MTOC targeting and assembly mediated by a septum-specific protein.
• The findings provide new insights into the regulation of microtubule organization in filamentous fungi.