Telomeres and centromeres have traditionally been considered to perform distinct roles. During meiotic prophase, in a conserved chromosomal configuration called the bouquet, telomeres gather to the nuclear membrane (NM), often near centrosomes. We found previously that upon disruption of the fission yeast bouquet, centrosomes failed to insert into the NM at meiosis I and nucleate bipolar spindles. Hence, the trans-NM association of telomeres with centrosomes during prophase is crucial for efficient spindle formation. Nonetheless, in approximately half of bouquet-deficient meiocytes, spindles form properly. Here, we show that bouquet-deficient cells can successfully undergo meiosis using centromere–centrosome contact instead of telomere–centrosome contact to generate spindle formation. Accordingly, forced association between centromeres and centrosomes fully rescued the spindle defects incurred by bouquet disruption. Telomeres and centromeres both stimulate focal accumulation of the SUN domain protein Sad1 beneath the centrosome, suggesting a molecular underpinning for their shared spindle-generating ability. Our observations demonstrate an unanticipated level of interchangeability between the two most prominent chromosomal landmarks.

In Saccharomyces cerevisiae, the PMT, KRE2/MNT1, and MNN1 mannosyltransferase protein families catalyze the steps of the O-mannosylation pathway, sequentially adding mannoses to target proteins. We have identified members of all three families and analyzed their roles in pathogenesis of the maize smut fungus Ustilago maydis. Furthermore, we have shown that PMT4, one of the three PMT family members in U. maydis, is essential for tumor formation in Zea mays. Significantly, PMT4 seems to be required only for pathogenesis and is dispensable for other aspects of the U. maydis life cycle. We subsequently show that the deletion of pmt4 results in a strong reduction in the frequency of appressorium formation, with the few appressoria that do form lacking the capacity to penetrate the plant cuticle. Our findings suggest that the O-mannosylation pathway plays a key role in the posttranslational modification of proteins involved in the pathogenic development of U. maydis. The fact that PMT homologs are not found in plants may open new avenues for the development of fungal control strategies. Moreover, the discovery of a highly specific requirement for a single O-mannosyltransferase should aid in the identification of the proteins directly involved in fungal plant penetration, thus leading to a better understanding of plant–fungi interactions.

Fungal pathogenesis depends on accurate secretion and location of virulence factors which drive host colonization. Protein glycosylation is a common posttranslational modification of cell wall components and other secreted factors, typically required for correct protein localization, secretion and function. Thus, the absence of glycosylation is associated with animal and plant pathogen avirulence. While the relevance of protein glycosylation for pathogenesis has been well established, the main glycoproteins responsible for the loss of virulence observed in glycosylation-defective fungi have not been identified. Here, we devise a proteomics approach to identify such proteins and use it to demonstrate a role for the highly conserved protein disulfide isomerase Pdi1 in virulence. We show that efficient Pdi1 N-glycosylation, which promotes folding into the correct protein conformation, is required for full pathogenic development of the corn smut fungus Ustilago maydis. Remarkably, the observed virulence defects are reminiscent of those seen in glycosylation-defective cells suggesting that the N-glycosylation of Pdi1 is necessary for the full secretion of virulence factors. All these observations, together with the fact that Pdi1 protein and RNA expression levels rise upon virulence program induction, suggest that Pdi1 glycosylation is important for normal pathogenic development in U. maydis. Our results provide new insights into the role of glycosylation in fungal pathogenesis. Fungal pathogens require virulence factors to be properly secreted and localized to guarantee complete infection. In common with many proteins, virulence factors must be post-translationally modified by glycosylation for normal localization, secretion and function. This is especially important for virulence factors, which are mainly comprised of cell wall and secreted proteins. Aberrant glycosylation leads to a loss of virulence in both animal and plant pathogenic fungi. We have previously demonstrated that glycosylation is important for virulence of the corn smut fungus, Ustilago maydis. However, the glycoproteins involved and their specific roles in the infection process have not yet been reported. Here, we describe a proteomic assay designed to identify glycoproteins involved in plant infection. Using this method, we define the role of Pdi1 protein disulfide isomerase in virulence. Interestingly, abolishing Pdi1 N-glycosylation mimics Δpdi1 defects observed during infection, suggesting that Pdi1 N-glycosylation is required for the normal secretion of virulence factors. We hypothesize that Pdi1 N-glycosylation is necessary for maintaining proper effector protein folding during the infection process, especially in the harsh conditions found inside the maize plant.

The O-mannosyltransferase Pmt4 has emerged as crucial for fungal virulence in the animal pathogens Candida albicans or Cryptococcus neoformans as well as in the phytopathogenic fungus Ustilago maydis. Pmt4 O-mannosylates specific target proteins at the Endoplasmic Reticulum. Therefore a deficient O-mannosylation of these target proteins must be responsible for the loss of pathogenicity in pmt4 mutants. Taking advantage of the characteristics described for Pmt4 substrates in Saccharomyces cerevisiae, we performed a proteome-wide bioinformatic approach to identify putative Pmt4 targets in the corn smut fungus U. maydis and validated Pmt4-mediated glycosylation of candidate proteins by electrophoretic mobility shift assays. We found that the signalling mucin Msb2, which regulates appressorium differentiation upstream of the pathogenicity-related MAP kinase cascade, is O-mannosylated by Pmt4. The epistatic relationship of pmt4 and msb2 showed that both are likely to act in the same pathway. Furthermore, constitutive activation of the MAP kinase cascade restored appressorium development in pmt4 mutants, suggesting that during the initial phase of infection the failure to O-mannosylate Msb2 is responsible for the virulence defect of pmt4 mutants. On the other hand we demonstrate that during later stages of pathogenic development Pmt4 affects virulence independently of Msb2, probably by modifying secreted effector proteins. Pit1, a protein required for fungal spreading inside the infected leaf, was also identified as a Pmt4 target. Thus, O-mannosylation of different target proteins affects various stages of pathogenic development in U. maydis. The O-mannosyltransferase Pmt4 is essential for virulence of animal and plant pathogenic fungi. This protein attaches one mannose at serine/threonine residues of cell wall and secreted proteins modulating their location and function. Thus, the crucial role of Pmt4 in fungal pathogenic development is probably caused by a defective glycosylation of its target proteins altering host-fungus interaction. In this paper, we performed a screen for Pmt4 target proteins employing the fungus Ustilago maydis, which causes smut disease in maize plants. This allowed identifying novel Pmt4 target proteins having a crucial role on its virulence. One of these targets is the signalling mucin Msb2, a conserved protein which acts upstream of MAP kinase cascades in various fungi and regulates early pathogenic development in U. maydis. We propose that Pmt4-dependent glycosylation of the extracellular domain of Msb2 is required for Msb2 activity and hence pathogenic development of U. maydis. This is divergent to the situation in S. cerevisiae where the mannosylated extracellular region of Msb2p possesses a negative regulatory function. In addition, we demonstrate important roles of Pmt4 during later stages of plant infection and identified Pmt4 target proteins which could be responsible for the virulence defect of pmt4 mutants during tumor formation.