Fungal F-box proteins have a diverse role in pathogenicity

Most fungal F-box proteins, which have been studied today, were described initially in S. cerevisiae and a substantial part of our current knowledge of SCF-functionality derives from studies in the baker’s yeast. The genome of S. cerevisiae encodes 22 F-box proteins with Cdc4 and Grr1 as the best characterized so far. SCF^Cdc4 ligases for instance have been accounted for the turnover of several target proteins, which are involved in various processes like cell-cycle control, morphology, nutrient- and calcium-sensing and thereby emphasize the vast diversity of molecular mechanisms, which can be connected to a sin-gle F-box protein (Table 7) (Finley et al., 2012; Jonkers and Rep, 2009). However, these mechanisms are only partially conserved between fungal species. An SCF^Cdc4 target pro-tein, which is required for pseudohyphal growth in S. cerevisiae is the transcription factor Tec1 (Chou et al., 2004). The Cdc4 homolog in the human pathogenic yeast Candida albicans is also involved in a switch from yeast-like to hyphal growth, which is required for pathogenicity. But in contrast to baker’s yeast no elevated Tec1 levels were observed in ∆cdc4 mutants of C. albicans (Atir-Lande et al., 2005; Chin et al., 2013).

Only few studies have been made for F-box proteins in filamentous fungi and most of them concentrate on homologs of Grr1 from yeast, which is responsible for a variety of cellular functions like cell-cycle regulation, morphology, amino-acid sensing and the con-trol of glucose repression (Benanti et al., 2007; Finley et al., 2012; Guo et al., 2015; Skaar et al., 2009). Similar to the deletion of GRR1 in S. cerevisiae, which leads to a stabiliza-tion of G1 cyclins Cln1 and Cln2, resulting in continuous pseudohyphal growth (Flick and Johnston, 1991; Kishi and Yamao, 1998; Loeb et al., 1999), the deletion of GRR1 in C. albicans results in the stabilization of G1 cyclins Ccn1 and Cln3 and thus triggers a constitutive pseudohyphal growth (Butler et al., 2006; Li et al., 2006). The Grr1 counter-part in the pathogenic yeast Cryptococcus neoformans, Fbp1, was shown to be essential for sexual development as well, but also plays a crucial role for pathogenicity. But in con-trast to the yeasts S. cerevisiae and C. albicans, C. neoformans ∆fbp1 mutants showed a specific sensitivity against cell-membrane stress, whereas glucose sensing and carbon catabolite repression was not influenced (Liu et al., 2011).

Table 7: F-box proteins in fungal pathogens. Fungal F-box proteins and their homologs with a verified or proposed role for pathogenicity. Most studies were carried out on ho-mologous proteins of S. cerevisiae Cdc4 and Grr1, which are also included. Hoho-mologous proteins to Fbx15 of A. fumigatus were identified by NCBI-BLAST search and display similarities to Fbx15 of 24-32%, similar to Fbx15 homolog in N. crassa (see also Ta-ble 5).

fungal species

F-box proteins and their homologs in fungal pathogens (F-box directed functions are indicated in brackets)

Yeast Model

NP: not present; NA: not analyzed; CCR: carbon catabolite expression

114 Discussion

The Grr1 homolog in A. nidulans, GrrA, was shown to have a specific role during meiosis, since ∆grrA mutants were blocked at the last step of ascospore formation during sexual development but otherwise resemble the wild-type phenotype (Krappmann et al., 2006a). Although grrA is able to complement the ∆grr1 phenotype in yeast, both Grr1 homologs have distinct functions within their species, for the reason that S. cerevisiae

∆grr1 mutants are still able to form asci with mature ascospores (Purnapatre et al., 2005).

GrrA shares similar functionalities with Fbp1, the Grr1 homolog of the plant pathogenic fungi Fusarium graminearum and Fusarium oxysporum. The deletion of FBP1 led to a loss of virulence for both fungi (Han et al., 2007; Miguel-Rojas and Hera, 2013; 2016).

Like ∆grrA mutants of A. nidulans loss of FBP1 in F. graminearum, which also is a homothallic fungus, results in a loss of sexual reproduction. However, ∆fbp1 mutants of F. graminearum were not able to produce any fruiting bodies thus displaying a more drastic effect on sexual development. Another characteristic, which distinguishes Fbp1 from its counterparts in yeast and A. nidulans, is the fact that FBP1 of F. graminearum can only partially complement the ∆grr1 phenotype in S. cerevisiae. Although defects in the glucose repression system as well as in the suppression of pseudohyphal growth could be restored, the sporulation defects were not complemented by heterologous expression of FBP1 in S. cerevisiae ∆grr1 mutants (Han et al., 2007). More recently also the Grr1 equivalent of Magnaporthe oryzae, another plant pathogenic fungus, which causes rice blast disease has been characterized as MoGrr1 with essential roles for asexual development, oxidative stress response and pathogenicity (Guo et al., 2015).

These examples present the importance of Grr1 homologs for virulence of a variety of pathogenic fungi. However, the function of Grr1 seems only partially to be conserved between fungal species. During this work it has been shown that the Grr1 homolog of A. fumigatus, GrrA, has only minor effects on external stresses and is not required for virulence in a mouse model of invasive pulmonary aspergillosis. These results further indicate that conserved fungal F-box proteins do not necessarily resemble the same func-tionality for different species. This is further corroborated by a study on the filamentous fungi specific F-box protein Frp1 in different plant pathogenic fungi. Whereas the dele-tion of FRP1 in Fusarium oxysporum led to a loss of pathogenicity, F. graminearum

∆frp1 mutants were only impaired during root infection, but no negative effect on infec-tion capabilities of Botrytis cinerea could be observed (Duyvesteijn et al., 2005; Jonkers et al., 2011). Thus it might be worth to examine the distinct functions of F-box proteins in

different pathogenic fungi in order to identify new mechanisms, which are connected to virulence.