Mnt1p and Mnt2p of Candida albicans Are Partially Redundant α-1,2-Mannosyltransferases That Participate in O-Linked Mannosylation and Are Required for Adhesion and Virulence

Abstract

The MNT1 gene of the human fungal pathogen Candida albicans is involved in O-glycosylation of cell wall and secreted proteins and is important for adherence of C. albicans to host surfaces and for virulence. Here we describe the molecular analysis of CaMNT2, a second member of the MNT1-like gene family in C. albicans. Candida albicans is the major fungal pathogen of humans. This opportunistic pathogen can cause irritating superficial infections of the mucosa and serious life threatening systemic infections in the immunocompromised patient (1, 2). Invasive candidosis in hospitals now represents the third or fourth most common form of septicaemia (3, 4). The cell surface of C. albicans is the immediate point of contact between the fungus and host and plays vital roles in adhesion and immunomodulation of host responses, and it is a source of antigens (5-8). The outer cell wall layer is enriched in mannoproteins, which are embedded in a matrix of structural polysaccharides consisting of ␤-1,3-and ␤-1,6-linked glucan and chitin (9). This layer is important in adhesion to host surfaces and their subsequent colonization (10 -12). Both the protein and carbohydrate components of mannoproteins have been implicated in adhesion to the host (10, 13-15), although details of the nature of the ligands and receptors are still lacking. Hence, glycosylation of cell wall proteins is critical for host-fungal interactions and pathogenicity. Mnt2p also functions inKnowledge of glycosylation in Saccharomyces cerevisiae (16 -28) and information from the C. albicans genome data base has provided significant resources for the identification and analysis of glycosylation genes in C. albicans. Mannoproteins of S. cerevisiae and C. albicans contain both N-and O-linked oligosaccharides. The N-linked glycans, attached to asparagine residues of proteins, contain a conserved core structure and an elaborate, highly branched outer mannose chain that is specific to fungi and contains both acid-stable and acid-labile components (17,29,30). Glycosylation in C. albicans has its own relevance in investigations of the role of specific oligosaccharide moieties in host-fungal interactions. The acid-labile mannosylphosphate component, containing ␤-1,2-linked mannose, has been implicated in adhesion and recognition of phagocytic leukocytes, although mutants lacking this component have been shown to have normal interactions with macrophages (31). Both ␤-1,2-and ␣-1,2-linked mannan oligosaccharides have been implicated directly in adhesion functions (12,32).In C. albicans, O-glycans are linear oligosaccharides of one to five ␣-1,2-linked mannose residues (32-34). In S. cerevisiae an ␣-1,2-linked O-linked glycan is capped with one or two ␣-1,3-linked mannose residues (27). O-Glycosylation in S. cerevisiae is initiated in the endoplasmic reticulum where at least four of the seven-membered PMT gene family act to transfer mannose from dolichyl phosphate-activated mannose to serine or threonine (18,35,36). Evidently this step is essential, as certain combinations ...

Outer Chain N-Glycans Are Required for Cell Wall Integrity and Virulence of Candida albicans

“…The O-linked oligosaccharides, attached to serine or threonine, consist of a linear chain of one to five ␣1,2-linked mannose residues (12)(13)(14) and are known to be required for full virulence (14). The process of N-glycosylation has been studied extensively in Saccharomyces cerevisiae.…”

Comparison of transcription of multiple genes during mycelia transition to yeast cells of Paracoccidioides brasiliensis reveals insights to fungal differentiation and pathogenesis

“…Mannosyltransferases (och1 and mnt1) could be putatively related to the O-linked mannosylation of proteins, as observed in C. albicans. C. albicans mutants to either mnt1 or och1 showed hypersensitivity to cell wall perturbing agents, suggesting the proteins role in the cell wall maintenance [43,44]. Moreover, a novel transcript encoding to guanosine diphosphatase (gmd1) was detected during the dimorphic transition, whose product is known to regulate mannosylation of Nand O-linked oligosaccharides in Golgi complex [45].…”

“…The cns1 product is an essential component of the HSP90 complex, which is induced in heat shock response [40]. Mannosyltransferases (mnt1 and och1) orthologues are required for cell wall integrity/virulence and adhesion/virulence, respectively, in C. albicans [43,44]. In S. pombe, the mde10 product is essential for development of spore envelopes [52] evidencing its importance during differentiation process in the cell.…”

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.…”