Abstract
Asexual spores (conidia) are the infectious propagules of many pathogenic fungi, and the capacity to sense the host environment and trigger conidial germination is a key pathogenicity determinant. Germination of conidia requires the de novo establishment of a polarised growth axis and consequent germ tube extension. The molecular mechanisms that control polarisation during germination are poorly understood. In the dimorphic human pathogenic fungus Penicillium marneffei, conidia germinate to produce one of two cell types that have very different fates in response to an environmental cue. At 25 °C, conidia germinate to produce the saprophytic cell type, septate, multinucleate hyphae that have the capacity to undergo asexual development. At 37 °C, conidia germinate to produce the pathogenic cell type, arthroconidiating hyphae that liberate uninucleate yeast cells. This study shows that the p21-activated kinase pakA is an essential component of the polarity establishment machinery during conidial germination and polarised growth of yeast cells at 37 °C but is not required for germination or polarised growth at 25 °C. Analysis shows that the heterotrimeric G protein α subunit GasC and the CDC42 orthologue CflA lie upstream of PakA for germination at both temperatures, while the Ras orthologue RasA only functions at 25 °C. These findings suggest that although some proteins that regulate the establishment of polarised growth in budding yeast are conserved in filamentous fungi, the circuitry and downstream effectors are differentially regulated to give rise to distinct cell types.
Highlights
The generation of an axis of cell polarity is central to the activity of many cells and the establishment of a wide variety of cell morphologies
Pseudohyphal growth requires the initiation of polarised growth for cellular elongation and is under the control of two signaling pathways: a cyclic adenosine monophosphate/protein kinase A (PKA) pathway and a mitogen-activated protein kinase (MAPK) cascade
The molecular mechanisms that control spore germination are poorly understood and studies of the opportunistic fungal pathogen Penicillium marneffei have shown that a group of highly conserved signalling and cell polarity factors, known as small GTPases, play important roles in germination and other aspects of morphogenesis
Summary
The generation of an axis of cell polarity is central to the activity of many cells and the establishment of a wide variety of cell morphologies. The establishment of polarised growth requires selection of a site to which proteins and components of the cytoskeleton are recruited. Saccharomyces cerevisiae establishes a polarised axis of growth during the processes of budding cell division, schmoo formation during mating, and pseudohyphal growth. Pseudohyphal growth requires the initiation of polarised growth for cellular elongation and is under the control of two signaling pathways: a cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway and a mitogen-activated protein kinase (MAPK) cascade. The cAMP/PKA pathway is activated by a glucose/sucrose sensitive receptor Gpr1p, which activates the G protein Gpa2p (a subunit), which in turn is inhibited by the novel kelch-Gb subunits Gpb1/2p, a third subunit Gpg1p, and a negative regulator Rgs2p [2,3,4,5,6,7,8]. Ste20p activates the MAPK cascade by phosphorylating Ste11p (MAPKKK); Ste11p phosphorylates Ste7p (MAPKK), which activates the Kss1p MAPK [13,14,15]
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