MNN5 Encodes an Iron-Regulated α-1,2-Mannosyltransferase Important for Protein Glycosylation, Cell Wall Integrity, Morphogenesis, and Virulence in Candida albicans

Abstract

The cell walls of microbial pathogens mediate physical interactions with host cells and hence play a key role in infection. Mannosyltransferases have been shown to determine the cell wall properties and virulence of the pathogenic fungus Candida albicans. We previously identified a C. albicans ␣-1,2-mannosyltransferase, Mnn5, for its novel ability to enhance iron usage in Saccharomyces cerevisiae. Here we have studied the enzymatic properties of purified Mnn5 and characterized its function in its natural host. Mnn5 catalyzes the transfer of mannose to both ␣-1,2-and ␣-1,6-mannobiose, and this activity requires Mn 2؉ as a cofactor and is regulated by the Fe 2؉ concentration. An mnn5⌬ mutant showed a lowered ability to extend O-linked, and possibly also N-linked, mannans, hypersensitivity to cell wall-damaging agents, and a reduction of cell wall mannosylphosphate content, phenotypes typical of many fungal mannosyltransferase mutants. The mnn5⌬ mutant also exhibited some unique defects, such as impaired hyphal growth on solid media and attenuated virulence in mice. An unanticipated phenotype was the mnn5⌬ mutant's resistance to killing by the iron-chelating protein lactoferrin, rendering it the first protein found that mediates lactoferrin killing of C. albicans. In summary, MNN5 deletion impairs a wide range of cellular events, most likely due to its broad substrate specificity. Of particular interest was the observed role of iron in regulating the enzymatic activity, suggesting an underlying relationship between Mnn5 activity and cellular iron homeostasis.

Impact of Yeast Glycosylation Pathway on Cell Integrity and Morphology

“…On the other hand, no clear effect on the cell integrity was observed [53]. However, the role of Mnn5 protein (an iron regulated alpyha-1,2 mannosyltransferase) from C.albicans in cell wall integrity maintenance was demonstrated [54]. A mnn5 delta mutant contains a reduced amount of cell wall mannan and hypersensitivity to the cell wall damaging agents.…”

The cell wall galactomannan antigen from Malassezia furfur and Malassezia pachydermatis contains β-1,6-linked linear galactofuranosyl residues and its detection has diagnostic potential

“…Mannan, the outermost layer component of the wall, behaves as a potent antigen and has been utilized as the target for the diagnosis of invasive candidiasis. It has been shown that mannosyltransferase deletion mutants of C. albicans have reduced pathogenicity (Bai et al, 2006;Bates et al, 2006;Lengeler et al, 2008). Furthermore, it is known that there are many mannose recognition molecules, such as the mannose receptor, dectin-2, the dendritic-cellspecific intercellular adhesion molecule-3 (ICAM-3)-grabbing non-integrin (DC-SIGN), galectin-3 and mannanbinding protein (Willment & Brown, 2008).…”

Enzymatic Synthesis of New Oligosaccharides Using Mannosyltransferases from Candida Species and Their NMR Assignments

“…These studies indicated that some mannooligosaccharide side chains are correlated with the pathogenicity of the C. albicans cells. 7,24) However, there is no study for the substrate specificity of these enzymes especially for the C. albicans specific mannosyltransferases. To characterize these mannosyltransferases, we need to use the purified recombinant enzyme.…”

“…[2][3][4] Furthermore, gene deletion experiments demonstrated that the mannan is important for the adherence to host cells and overall virulence. [5][6][7] Mannan consists of a linear a-1,6-linked mannose polymer backbone with side chains composed of a-1,2-, a-1,3-, a-1,6-, and b-1,2-linked mannoses. 8) The side chain mannooligosaccharide sequences are involved in the adhesion or attachment of C. albicans to the macrophage, dendritic cell, or epithelial cell through binding to the mannose-receptor, dectin-2, the dendritic-cell-specific intercellular adhesion molecule-3 (ICAM-3)-grabbing non-integrin (DC-SIGN), and mannan-binding protein.…”