A Multifunctional Mannosyltransferase Family in Candida albicans Determines Cell Wall Mannan Structure and Host-Fungus Interactions

Abstract

ThecellwallproteinsoffungiaremodifiedbyN-andO-linkedmannosylation and phosphomannosylation, resulting in changes to the physicalandimmunologicalpropertiesofthecell.Glycosylationofcell wall proteins involves the activities of families of endoplasmic reticulum and Golgi-located glycosyl transferases whose activities are difficult to infer through bioinformatics. The Candida albicans MNT1/ KRE2 mannosyl transferase family is represented by five members. We showed previously that Mnt1 and Mnt2 are involved in O-linked mannosylation and are required for virulence. Here, the role of C. albicans MNT3, MNT4, and MNT5 was determined by generating single and multiple MnT⌬null mutants and by functional complementation experiments in Saccharomyces cerevisiae. CaMnt3, CaMnt4, and CaMnt5 did not participate in O-linked mannosylation, but CaMnt3 and CaMnt5 had redundant activities in phosphomannosylation and were responsible for attachment of approximately half of the phosphomannan attached to N-linked mannans. CaMnt4 and CaMnt5 participated in N-mannan branching. Deletion of CaMNT3, CaMNT4, and CaMNT5 affected the growth rate and virulence of C. albicans, affected the recognition of the yeast by human monocytes and cytokine stimulation, and led to increased cell wall chitin content and exposure of ␤-glucan at the cell wall surface. Therefore, the MNT1/KRE2 gene family participates in three types of protein mannosylation in C. albicans, and these modifications play vital roles in fungal cell wall structure and cell surface recognition by the innate immune system.The human pathogen Candida albicans is the most frequent cause of systemic candidosis, which is a common, life-threatening infection in immunocompromised patients (1). The C. albicans cell wall is a robust yet dynamic structure that protects the cell from changes in the extracellular environment. It is the immediate contact point with host cells and contains antigenic determinants, glycoproteins involved in the adhesion to host tissues, and most of the pathogen-associated molecular patterns that are recognized by host immune system (2). The wall is organized in an inner skeletal layer comprising chitin, ␤1,3-and ␤1,6-glucans, and an outer layer that is dominated by highly glycosylated proteins (3). These proteins are post-translationally modified with N-and/or O-linked mannans, both of which can be further elaborated with oligomannosides that are attached via phosphodiester linkages (phosphomannans). Mannans have important roles in cell wall integrity, adhesion to host cells and tissues, virulence, and the establishment of a response by immune cells (2, 4 -10). The O-and N-linked mannans, along with ␤-glucans, represent the main C. albicans pathogen-associated molecular patterns recognized by the innate immune system (2, 11-13).Mannan biosynthesis has been carefully characterized in Saccharomyces cerevisiae, and the main features of the pathways involved in the construction of these oligosaccharides are conserved in C. albicans. However, N-and O-linked mannans of C. al...

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

“…Then, the O-linked glycans are further elongated by members of the KRE2/MNT1 gene family which, as in N-linked glycosylation, adds ␣1,2-mannose residues, forming linear ␣1,2-mannose polymers that in Trichoderma can contain up to three monosaccharide units (565,578). Analysis of S. cerevisiae and C. albicans KRE2/MNT1 gene family members indicated that Trichoderma KTR1 and KRE2 orthologs are likely to be involved in the O-linked glycosylation pathway (568,579,580). Since some family members are quite promiscuous in terms of substrate specificity, and the functional roles of S. cerevisiae Ktr4 and Ktr5 have not been clearly established (568), it is possible that the orthologs found in Trichoderma could also participate in the biosynthesis of these glycans.…”

Posttranslational Modifications of Proteins in the Pathobiology of Medically Relevant Fungi

“…Numerous cell surface proteins are posttranslationally modified in fungi, and disruption of these modifications can significantly attenuate virulence. For example, cell wall and secreted mannoproteins are highly glycosylated in S. cerevisiae and Candida albicans (6,113) (Table 1). The importance of N-glycosylation for fungal pathogenesis has been confirmed, for example, by disruption of the ␣1,6-mannosyltransferase Och1 (6).…”

N-Glycosylation of Effector Proteins by an α-1,3-Mannosyltransferase Is Required for the Rice Blast Fungus to Evade Host Innate Immunity

“…Deletion of some ALG genes involved in the earliest steps of N-glycosylation results in lethal, or extremely severe, phenotypes and the secretion of underglycosylated proteins, demonstrating the biological importance of N-linked glycosylation (Kelleher and Gilmore, 2006). In Candida albicans, N-glycosylation is important for both the structure and function of certain proteins that play vital roles in cell-cell interactions, adhesion, and reaction to host immune responses (Poulain and Jouault, 2004;Mora-Montes et al, 2010;Hiller et al, 2011). However, the role of N-glycosylation in the pathogenesis of filamentous fungi remains largely unexplored.…”