Wednesday March 31

Parallel session 6: Fungal Way of Living: Cell Biology

PS6.1

Ancestral homologs of the yeast bud site selection proteins regulate septum formation and development in filamentous fungi

Steven Harris, Haoyu Si

University of Nebraska

sharri1@unlnotes.unl.edu

The defining feature of fungal cells is polarized growth, whereby cell wall deposition is confined to a discrete location on the cell surface.  The annotation of multiple fungal genome sequences has revealed that the signaling modules and morphogenetic machinery involved in polarized growth are largely conserved across the fungal kingdom.  Nevertheless, fungal cells exhibit a diverse variety of shapes that are largely based on two growth patterns: hyphae and yeast.  We suggest that these different patterns reflect variation in the mechanisms that spatially and temporally regulate cellular morphogenesis. To test our hypothesis, we are characterizing ancestral homologues of the yeast bud site selection proteins. In particular, we have found that the yeast axial bud pattern markers Bud3, Bud4, and Axl2 are weakly conserved in the Pezizomycotina.  Functional studies in Aspergillus nidulans implicate Bud3 as a guanine nucleotide exchange factor (GEF) that regulates septation in hyphae by activating the GTPase Rho4. Bud4 is also involved in septum formation; genetic interactions suggest that it might facilitate septin organization. In addition, our studies reveal roles for Bud4 and Axl2 in cytokinesis during conidiophore development. Notably, Bud4 localizes to all septa in conidiophores, whereas Axl2 is only found at the junction between spores and their subtending phialide. Our observations support the existence of a phialide-specific morphogenetic program that might be unique to the Aspergilli and related Eurotiomycetes. Furthermore, they also provide insight into the ancestral functions of the yeast bud site selection system in the filamentous fungi.

PS6.2

Biogenesis and evolution of the fungal Woronin body

Gregory Jedd

Temasek Life Sciences Laboratory and Department of Biological Sciences, The National University of Singapore, Singapore

greg@tll.org.sg

Woronin bodies are peroxisome-derived organelles that evolved approximately 500 million years ago in a common ancestor of filamentous Ascomycetes where they perform an adaptive function supporting the hyphal syncytium. These organelles are centered on the HEX protein, which self-assembles to produce micrometer scale protein assemblies that bud from the peroxisome to produce a second organelle with a distinct composition and cellular localization. Forward genetic screens in Neurospora crassa have identified two new genes, wsc (Woronin sorting complex) and leashin, which encode key components of the WB biogenesis machinery. WSC functions by forming membrane associated oligomers that envelop HEX assemblies to promote budding.  In a second function, WSC engages the cytoplasmic tethering protein Leashin, which mediates cell cortex association in a step that is essential for organelle inheritance. This work defines a biogenesis pathway in which the dual function of WSC acts to coordinate organelle morphogenesis and inheritance.  I will conclude my talk with a discussion of mechanisms that control Woronin body abundance and speculation on the nature of genetic innovation that fostered Woronin body evolution.

PS6.3

The role of microbodies in penicillin production

L. Gidijala, J.A.K.W. Kiel, M. Veenhuis, I. J. van der Klei

Molecular Cell Biology, Groningen Biomolecular and Biotechnology Institute, University of Groningen, PO Box 14,  Kerklaan 30, 9750 AA Haren, The Netherlands

i.j.van.der.klei@rug.nl

The filamentous fungus P. chrysogenum is the industrial producer of the important β-lactam antibiotic penicillin. The initial steps of the penicillin biosynthetic pathway are localized in the cytosol, namely the non-ribosomal peptide synthetase δ-(L-aminoadipyl)-L-cysteinyl-D-valine synthetase (ACVS), which produces the tripeptide ACV, and isopenicillin N synthetase (IPNS), which catalyses the formation of isopenicillin N (IPN). The last steps of penicillin biosynthesis occur in specialized organelles, the microbodies (peroxisomes). These organelles contain the enzymes isopenicillin N:acyl CoA acyltransferase (IAT) and phenylacetyl-CoA ligase (PCL), which catalyze the conversion of IPN into penicillin G.

The peroxisomal localization of IAT and PCL in peroxisomes is essential for efficient penicillin production, because mutants defective in peroxisome formation show reduced penicillin production levels. Moreover, a close correlation seem to exist between penicillin production and the volume fraction of peroxisomes per cell [1]. Additionally, artificial overproduction of a single peroxisomal membrane protein, Pex11p, resulted in increased penicillin production levels together with massive proliferation of peroxisomes. In this strain the level of the penicillin biosynthetic enzymes was not altered [2]. Recently we introduced the penicillin biosynthesis pathway into the yeast Hansenula polymorpha. In this organism penicillin was produced and efficiently secreted in the medium. Also in this heterologous host peroxisome deficiency resulted in strongly decreased penicillin production levels [3].

References:
1. van den Berg, M.A., et al., Nat Biotechnol 26, 1161-8. 2008
2. Kiel, J.A., et al., Fungal Genet Biol 42, 154-64. 2005
3. Gidijala L., et al., PLoS One 4(12):e8317. 2009


 

References:

1.            van den Berg, M.A., et al., Nat Biotechnol 26, 1161-8. 2008

2.            Kiel, J.A., et al., Fungal Genet Biol 42, 154-64. 2005

3.            Gidijala L., et al., PLoS One 4(12):e8317. 2009

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