Aspergillus nidulans
Role of Myosin II in Septum Formation

In our work with Aspergillus nidulans type II myosin (MyoB), we discovered a temperature-sensitive mutation in the myoB gene which blocks septum formation.  The predicted amino acid change is in a critical region of the converter subdomain of the myosin head.  Myosin rings form at septation sites in the mutant strain but fail to contract normally.  The following diagram, taken from Hill et al. 2015, contrasts normal myosin ring contraction (panels D - G) with abnormal ring formation in the mutant strain (panels H - K).   In each set, the first three images are a time-lapse series and the last panel (G or K) displays those same three time points in false colors in order to contrast the symmetry of wild type ring contraction (D - F) with the assymetry of mutant ring contraction (H - J).   Videos S1 and S2 show ring contraction of wild type and mutant MyoB rings, respectively.

Movie S1

Movie S2

By repressing expression of the myoB gene under the regulatable AlcA promoter, we also demonstrate that actin rings still form at septation sites even under condtions of reduced levels of wild type MyoB, but ring constriction and septum formation is prevented.   In Movie S3 (below), actin is imaged in a myoB-repressed cell using an in vivo actin-binding Lifeact::mRFP construct.   Note that actin rings form, but they fail to organize well and eventually disassemble without contracting.

Movie S3

Conversely, when filamentous actin is inhibited by latrunculin, MyoB rings fail to form.

We therefore conclude that actin does not require normal levels of myosin for basic actin ring assembly, but normal levels of myosin are required for actin ring contraction.   Myosin, however, requires the presence of actin in order to participate in contractile ring formation.

Literature:

Hill, T. W, L. Jackson-Hayes, X. Wang, and B. L. Hoge.   2015.  "A mutation in the converter subdomain of Aspergillus nidulans MyoB blocks constriction of the actomyosin ring in cytokinesis."  Fungal Genetics & Biology 75: 72-83.

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