The SEPH protein controls cytokinesis in Aspergillus nidulans
Abstract
In the filamentous fungus Aspergillus nidulans, multiple rounds of nuclear division occur prior to cytokinesis. The SepH gene from A. nidulans was discovered in a screen for temperature-sensitive cytokinesis mutants. Sequence analysis showed that SEPH is 42% identical to the serine/threonine kinase Cdc7p from fission yeast. Signaling through the Septation Initiation Network (SIN), which includes Cdc7p and the GTPase Spg1p, is emerging as a primary regulatory pathway used by fission yeast to control cytokinesis. In addition to SEPH, potential homologues were also identified in other fungi and plants but not in animal cells. Deletion of SepH resulted in a viable strain that failed to septate at any temperature and the inability to form proper asexual structures called conidiophores. Although no conidia are formed, the basic morphological changes that define conidiophore development still occur, which indicates that cellularization is not required for differentiation in A. nidulans. Quantitative analysis of the actin cytoskeleton revealed that SepH is required for construction of the actin ring. High levels of an epitope tagged SEPH protein fusion under the control of the inducible AlcA promoter results in the formation of multiple aberrant septa. A strain that contains a SepH-Green Fluorescent Protein (GFP) gene fusion behind the AlcA promoter is capable of complementing the sepH1 allele. When grown in the presence of threonine (highest induction), pattern of brightly fluorescent foci or spots emerges near nuclei. Analysis of a large number of cells grown with a low level of induction stained for microtubules revealed that these foci are never seen in mitotic cells. However, GFP fluorescence was observed at the ends of mitotic spindles in a very small number of cells. These observations suggest that SepH may only localize to spindle pole bodies in a subset of cells. This subset may be limited to cells that are about to septate and that are in late mitosis. I conclude that A. nidulans has components of a SIN pathway, one of which, SEPH, is required for early events during cytokinesis.
Degree
Ph.D.
Advisors
Staiger, Purdue University.
Subject Area
Cellular biology|Microbiology
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