Could be related to calcium signalling
E values for the BLASTP analysis and the amino acid sequence identity between the M. oryzae protein and the predicted polypeptide sequence for the E.festucae homologue are given. Proteins chosen for this analysis were based on those present in the M. oryzae genome (Nguyen et al. 2008). Other calcium signalling genes may be present in the E. festucae. E. festucae gene models are available on request
E values for the BLASTP analysis and the amino acid sequence identity between the M. oryzae protein and the predicted polypeptide sequence for the E.festucae homologue are given. Proteins chosen for this analysis were based on those present in the M. oryzae genome (Nguyen et al. 2008). Other calcium signalling genes may be present in the E. festucae. E. festucae gene models are available on request the cellular response to stress induced by high salt concentration (Cunningham and Fink 1996; Park et al. 2001). PMR1 homologues in A. niger and Aspergillus fumigatus are required for maintaining Ca2+ homeostasis in these fungi (Yang et al. 2001; Pinchai et al. 2010). However, A. fumigatus pmrA, encoding a Ca2+ ATPase, is not required for pathogenicity (Pinchai et al. 2010). E.festucae contains three PMC1 (vacuolar Ca2+ ATPases) and two PMR1 (Golgi Ca2+ ATPases) homologues, and five Ca2+ exchangers (Table 4).
In S. cerevisiae, deletion of PLC1 was shown to have pleiotropic effects, including variable rates of survival depending on the genetic background (Yoko-o et al. 1993). Mutants display sensitivity to osmotic stress, temperature stress and show defects in the utilisation of carbon sources other than glucose (Flick and Thorner 1993). PLC1 works together with the Gpr1p/Gpa2p GPCR to transduce glucose-induced calcium signals (Flick and Thorner 1993; Tisi et al. 2002). In M. oryzae, disruption of plc1 resulted in reduced pathogenicity, presumably due to disturbance of calcium fluxes essential for the infection process (Rho et al. 2009). E. festucae contains four PLC homologues (Table 4). Given the pathogenicity defects of PLC1 mutants of M. oryzae the homologue from E. festucae may be important for signalling and maintenance of the symbiosis.
The calcium sensor calmodulin is involved in many cell processes through the regulation of targets such as the Ca2+/calmodulin-dependent protein phosphatase, calcineurin or Ca2+/calmodulin dependent protein kinases. The S. cerevisiae calmodulin gene, CMD1, is essential for viability (Davis et al. 1986). While little is known about the role calmodulin plays in phytopathogenicity, expression of M. oryzae calmodulin is upregulated during appressorium formation (Liu and Kolattukudy 1999). Additionally, calmodulin preferentially localises to germ tubes and appressoria, suggestive of a role in the infection process (Ma et al. 2009). In support of this, silencing of calmodulin was found to dramatically reduce pathogenicity of M. oryzae. E. festucae contains one, highly conserved CAM homologue (Table 4).
Ca2+/Calmodulin-Dependent Protein Kinases
An important group of calcium-regulated enzymes is the Ca2+/calmodulin-depen-dent protein kinases (CaMKs). CaMKs are serine/threonine protein kinases with an amino terminal catalytic domain and a carboxy-terminal regulatory domain. The regulatory domain consists of an autoinhibitory region and the Ca2+/calmodulin (Ca2+/CaM) binding region. The autoinhibitory domain regulates the activity of the enzyme by interacting with the catalytic domain to prevent substrate binding or distort the catalytic site. Binding of Ca2+ activated calmodulin with the calmodulin-binding domain results in a conformational change that releases the catalytic domain to overcome this autoinhibition. After the initial activation, the enzyme may remain active independently of the presence of Ca2+/CaM. Furthermore, the enzyme may require additional modifications, such as phosphorylation, for full activation. Fungal CaMKs play important roles in fungal development as well as interactions with other organisms. S. cerevisiae contains two CaM kinases, CMK1 and CMK2 (Ohya et al. 1991). These are involved in thermotolerance (Iida et al.
1995) and cell survival following pheromone-induced cell cycle arrest (Moser et al.
1996). In the filamentous fungus A. nidulans, three Ca2+/CaM-dependent protein kinases, CmkA, CmkB and CmkC, have been reported. CmkA and CmkB are bona fide CaMKs, whereas CmkC is a CaMKK, which phosphorylates CmkB in vitro. Disruption of cmkA or cmkB is lethal, while expression of a constitutively active form of CmkA prevents spores from entering the first nuclear division cycle (Dayton and Means 1996; Dayton et al. 1997; Joseph and Means 2000). In N. crassa, deletion of CAMK-1 induces a transient slow growth phenotype (Yang et al. 2001). CaM kinases also appear to play a role in phytopathogenicity. M. oryzae cmk1 mutants have delayed spore germination and appressorium formation, leading to a reduced ability to infect host plants (Liu et al. 2009). Additionally, inhibitors of CaMK block the Colletotrichum gloeosporioides infection process (Kim et al. 1998). However, not all CaM kinases are essential for phytopatho-genicity, as loss of any of the three CaMKs of Stagnospora nodorum does not prevent pathogenicity (Solomon et al. 2006). E. festucae contains homologues of A. nidulans cmkA, cmkB and cmkC (Table 4). However, none of these genes appear to be required for establishment or maintenance of the symbiotic interaction between E. festucae and perennial ryegrass (Mitic 2011).
Calcineurin is a Ca2+/calmodulin-dependent protein phosphatase (PP2B) comprised of a catalytic subunit (CnA) and a regulatory subunit (CnB). The catalytic subunit is a polypeptide with four different domains: catalytic domain, CnB binding domain, Ca2+/CaM binding domain and an autoinhibitory domain. As for CaMKs, the autoinhibitory domain inhibits enzyme activity and this inhibition is relieved by Ca2+/CaM binding (Rusnak and Mertz 2000). Calcineurin signalling, which occurs via the calcineurin-responsive transcription factor CRZ1, is important for regulation of ion stress, cell wall integrity, hyphal growth and a number of other developmental processes (Stie and Fox 2008). Calcineurin signalling is also important for many fungal-plant associations. In U. maydis, calcineurin is essential for virulence, with mutants unable to form tumours (Egan et al. 2009). In M. oryzae, treatment with the calcineurin inhibitor cyclosporin A blocks appressorium formation, and RNAi silencing of the calcineurin A gene, MCNA, significantly reduces appressorium formation (Choi et al. 2009b). Disruption of M. oryzae CRZl confirmed that calcineurin signalling is required for the infection process, as mutants seldom developed infectious hyphae (Choi et al. 2009a), possibly due to the reduced turgor pressure of the mutant compared to wild type (Zhang et al. 2009). B. cinerea crzl mutants are defective in penetration, but this can be rescued by addition of Mg2+ (Schumacher et al. 2008). An analysis of the genome sequences of E. festucae strains E2368 and E984 (Fl1) identified two copies of cnaA in the former and just one in the latter (Table 4). A cnaA deletion mutant of Fl1 induced a severe HR-like response when seedlings of perennial ryegrass were infected with this mutant (Mitic 2011), suggesting calcineurin signalling does play a role in symbiotic maintenance.
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