Academic journal article Genetics

Regulation of Septum Formation by the Bud3-Rho4 GTPase Module in Aspergillus Nidulans

Academic journal article Genetics

Regulation of Septum Formation by the Bud3-Rho4 GTPase Module in Aspergillus Nidulans

Article excerpt

ABSTRACT

The ability of fungi to generate polarized cells with a variety of shapes likely reflects precise temporal and spatial control over the formation of polarity axes. The bud site selection system of Saccharomyces cerevisiae represents the best-understood example of such a morphogenetic regulatory system. However, the extent to which this system is conserved in the highly polarized filamentous fungi remains unknown. Here, we describe the functional characterization and localization of the Aspergillus nidulans homolog of the axial bud site marker Bud3. Our results show that AnBud3 is not required for polarized hyphal growth per se, but is involved in septum formation. In particular, our genetic and biochemical evidence implicates AnBud3 as a guanine nucleotide exchange factor for the GTPase Rho4. Additional results suggest that the AnBud3-Rho4 module acts downstream of the septation initiation network to mediate recruitment of the formin SepA to the site of contractile actin ring assembly. Our observations provide new insight into the signaling pathways that regulate septum formation in filamentous fungi.

THE filamentous fungi form mycelial colonies that consist of networks of branched hyphae that grow by apical extension. In the higher fungi (i.e., Ascomycota and Basidiomycota), hyphae are compartmentalized by the formation of cross-walls, or septa. It has long been suspected that the presence of septa allows filamentous fungi to partition cellular environments within a hypha to support colony homeostasis and reproductive development (Gull 1978). The process of septum formation is similar to cytokinesis of animal cells, in that it coordinated with mitosis and requires formation of a contractile actin ring (CAR) (Balasubramanian et al. 2004). By analogy to the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, the CAR likely provides a landmark that guides deposition of the septal wall material. However, unlike these yeasts, the septum is not subsequently degraded and cells remain attached. Furthermore, in most filamentous fungi, a small pore is retained to enable communication between adjacent hyphal compartments. Septum formation has been studied in several filamentous fungi, including Aspergillus nidulans (Harris 2001; Walther and Wendland 2003). Upon germination of asexual conidiospores in A. nidulans, the first few rounds of parasynchronous nuclear division are not accompanied by septation until cells reach an appropriate size/ volume (Harris et al. 1994; Wolkow et al. 1996). Subsequently, the first septum forms near the junction of the spore and germ tube (Harris et al. 1994). Deposition of the septal wall material is tightly coupled to assembly and constriction of the CAR, which in turn requires persistent signals from mitotic nuclei (Momany and Hamer 1997). As A. nidulans hyphae continue to grow by apical extension, each parasynchronous round of mitosis in multinucleate tip cells is followed by formation of septa in the basal region of the compartment (Clutterbuck 1970). Because tip and intercalary hyphal cells are multinucleate, not all of the individual mitotic events within the tip cell are capable of triggering septation.

Genetic analyses have identified several functions required for septumformation in A. nidulans, including the septation initiation network (SIN), the septins, and a formin (Harris 2001). The SIN is a cascade of three protein kinases that is activated by a small GTPase (Krapp and Simanis 2008). In A. nidulans, the component kinases of the SIN are arranged in the pathway SepH[arrow right]SepL[arrow right]SidB, with SepM amd MobA serving as cofactors that regulate SepL and SidB, respectively (Kim et al. 2006, 2009). Although SIN components localize to the spindle pole bodies, this does not appear to be a prerequisite for their subsequent recruitment to the septation site (Kim et al. 2009). Functional analysis of SepH, ModA, and SidB demonstrate that the SIN is required for assembly of the CAR (Bruno et al. …

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