Aspergillus nidulans
Role of IQGAPs in Septum Formation

One of the classic septation-deficient mutations in Aspergillus nidulans is the temperature-sensitive sepG1 mutation, whose gene locus, despite the mutation's having been discovered as long ago as 1994, had not (until now) been identified.   Through Mendelian gene mapping and sequencing of genomic DNA, we have demonstrated that the sepG1 mutation lies in the A. nidulans orthologue encoding the multi-functional scaffolding protein IQGAP.   By deleting the IQGAP gene and by down-regulating its expression via introduced promoters, we have demonstated that the IQGAP protein is necessary for constriction of the contractile actomyosin ring during cytokinesis.

The top image depicts the domain structure of A. nidulans IQGAP, where G/R designates the predicted amino acid substitution. E = Normal distribution of septal crosswalls; F = Absence of crosswalls after IQGAP gene deletion; G = Absence of crosswalls after gene down-regulation

Among several other things demonstrated regarding A. nidulans IQGAP is evidence that it binds to the EF-hand protein Cdc4 (myosin essential light chain) but not to EF hand proteins calmodulin (CamA) or myosin regulatory light chain (MrlC). IQGAP does not, however, require Cdc4 in order to localize to sites of septation.

C = IQGAP at septation site; D = Cdc4 at septation site; H = Localization of IQGAP in the absence of Cdc4; I = The same cell stained to show failure to form septa in the absence of Cdc4

Literature:

Hill, T. W, K. E. Wendt, D. A. Jones, M. H. Williamson, U. J. Ugwu, L. B. Rowland, and L. Jackson-Hayes.   The Aspergillus nidulans IQGAP orthologue SepG is required for constriction of the contractile actomyosin ring.  Fungal Genetics & Biology 144: 103439.

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