The Genetics and Biochemistry of Cell Wall Structure and Synthesis in Neurospora crassa, a Model Filamentous Fungus

Abstract

This review discusses the wealth of information available for the N. crassa cell wall. The basic organization and structure of the cell wall is presented and how the wall changes during the N. crassa life cycle is discussed. Over forty cell wall glycoproteins have been identified by proteomic analyses. Genetic and biochemical studies have identified many of the key enzymes needed for cell wall biogenesis, and the roles these enzymes play in cell wall biogenesis are discussed. The review includes a discussion of how the major cell wall components (chitin, β-1,3-glucan, mixed β-1,3-/ β-1,4- glucans, glycoproteins, and melanin) are synthesized and incorporated into the cell wall. We present a four-step model for how cell wall glycoproteins are covalently incorporated into the cell wall. In N. crassa, the covalent incorporation of cell wall glycoproteins into the wall occurs through a glycosidic linkage between lichenin (a mixed β-1,3-/β-1,4- glucan) and a “processed” galactomannan that has been attached to the glycoprotein N-linked oligosaccharides. The first step is the addition of the galactomannan to the N-linked oligosaccharide. Mutants affected in galactomannan formation are unable to incorporate glycoproteins into their cell walls. The second step is carried out by the enzymes from the GH76 family of α-1,6-mannanases, which cleave the galactomannan to generate a processed galactomannan. The model suggests that the third and fourth steps are carried out by members of the GH72 family of glucanosyltransferases. In the third step the glucanosyltransferases cleave lichenin and generate enzyme/substrate intermediates in which the lichenin is covalently attached to the active site of the glucanosyltransferases. In the final step, the glucanosyltransferases attach the lichenin onto the processed galactomannans, which creates new glycosidic bonds and effectively incorporates the glycoproteins into the cross-linked cell wall glucan/chitin matrix.

Chitosan and postharvest decay of fresh fruit: Meta‐analysis of disease control and antimicrobial and eliciting activities

“…Chitosan application to plants fits into the delicate relationship between the host and pathogenic fungi and involves the primary cell‐wall defense mechanisms. A link between pathogenicity and the enzymes that synthesize the fungal cell wall has been demonstrated in numerous studies (Arana et al., 2009; Geoghegan et al., 2017; Lenardon, Munro, & Gow, 2010; Levdansky, Kashi, Sharon, Shadkchan, & Osherov, 2010; Oliveira‐Garcia, & Deising, 2013; Patel & Free, 2019), and depolymerization of the cell walls of plant pathogenic fungi following the infection, evading plant immune recognition, has been reported (Geoghegan et al., 2017). It has been reported that the strategy of some fungal pathogens to evade plant immunity is to convert chitin into chitosan (Lopez‐Moya et al., 2019).…”

“…There are direct links between the cell wall and cell membranes, as the synthesis of key cell‐wall components (e.g., glucans, chitin) occurs at the plasma membrane, with the associated synthase enzyme complexes (Maddi, & Free, 2010). The chitin is localized in the membrane proximal portion of the cell wall and is incorporated into the wall matrix by being cross‐linked to the glucans (Patel & Free, 2019). Previous studies have investigated the role of plasma membrane in the sensitivity of fungi to chitosan showing that the plasma membrane of chitosan‐sensitive fungi is more fluid and richer in polyunsaturated free fatty acids than in chitosan‐resistant fungi (Palma‐Guerrero et al., 2009, 2010).…”

A seven-membered cell wall related transglycosylase gene family in Aspergillus niger is relevant for cell wall integrity in cell wall mutants with reduced α-glucan or galactomannan

“…A closer related species to T. harzianum and a Sodariomycete, N. crassa , revealed a BLAST-based orthologue NCU04431 to gluc31 (96% query coverage, 56.74% identity), also encoding a GH16 member. A knockout of NCU04431 was recently shown to exhibit a wild type growth morphology ( Patel and Free, 2019 ), which may suggest that gluc31 only plays a specific role in cell wall biogenesis of T. harzianum . Taken together with transcriptional data from A. niger , we do not expect that other non-Crh GH16 enzymes of A. niger act in chitin to glucan transglycosylation.…”

“…Neurospora crassa and Aspergillus fumigatus both encode five orthologues, whereas A. niger contains seven orthologues ( Pel et al, 2007 ). All these three fungi displayed wild type growth morphology for all single knockouts strains ( Fang et al, 2019 , Patel and Free, 2019 , van Leeuwe et al, 2019 ). In case of A. fumigatus, Af Crh5p was structurally and functionally characterized as a true transglycosylase with both homo- and heterotransglycosylation activity, additionally identifying both donor and acceptor substrate binding kinetics and moieties ( Fang et al, 2019 ).…”

Genome Wide Analysis Reveals the Role of VadA in Stress Response, Germination, and Sterigmatocystin Production in Aspergillus nidulans Conidia

“…Chitin and β-glucan are the primary polysaccharides involved in cell wall integrity in most fungi [ 9 , 43 ]. There are dynamic levels of chitin and β-glucan biosynthesis during hyphal tip extension [ 44 , 45 ]. In conidia, the VosA-VelB complex negatively regulates the expression of various genes, including cell wall-related genes, resulting in the repression of chitin and β-glucan biosynthesis [ 24 ].…”