How branches are added to the linear β-1,3-glucan backbones

The gap

Evidence profile

Sourced from the future work and stated research gap of the source papers, classified as general, drawn from work published between 2024 and 2026, spanning 3 journals. Those papers have been cited 18 times in total.

Supporting evidence — 3 representative gaps

• The Structure, Biosynthesis, and Function of β-1,6-Glucan in the Fungal Cell Wall (2026) · Biomolecules · doi

  5.1. Perspectives on Biosynthesis and Regulation of β-1,6-Glucan In this review, we have systematically integrated the current understanding of the structure, biosynthesis, and function of β-1,6-glucan in the fungal cell wall. Structurally, https://doi.org/10.3390/biom16091233 Biomolecules 2026, 16, 1233 15 of 21 β-1,6-glucan is a quantitatively typically minor but architecturally indispensable polysac- charide that functions as a covalent cross-linker, tethering the external mannoprotein layer to the internal chitin–β-1,3-glucan skeletal network, and thereby constituting a mainstay of cell wall integrity and plasticity. Although its absolute abundance and chain architecture differ markedly among species, its fundamental role as the structural bridge of the bilayer cell wall is invariably conserved. Biosynthetically, unlike chitin and β-1,3-glucan, which are polymerized by single plasma membrane-associated synthases, β-1,6-glucan synthesis depends on a multi-protein, multi-compartment cooperative network spanning the ER (Kre5, Big1, Cwh41/Gls1, Rot2/Gls2, and Cne1), the Golgi apparatus (Kre6/Skn1 family homologs), and the cell surface (Kre9, Knh1, Kre1, and Kre11); in vitro reconstitution has further established that polymerization employs UDP-glucose as the direct sugar donor and is positively regulated by the small GTPase Rho1, while strictly depending on preserved cellular integrity. Functionally, two complementary lines of evidence, enzymatic digestion by β-1,6-glucanases and genetic inactivation of the biosynthetic machinery, jointly demon- strate that β-1,6-glucan is essential for cell wall integrity, proper GPI-anchored protein localization, hyphal growth, morphogenesis, and fungal virulence, while simultaneously acting as a potent immunomodulatory signaling molecule at the fungus–host interface. Taken together, these studies establish β-1,6-glucan as a multifunctional molecule with a triad of structural, developmental, and immunomodulatory roles, underscoring its central importance in fungal cell wall biology and host–pathogen interactions. Based on the cumulative genetic, biochemical, and cell biological evidence reviewed herein, we propose a multi-compartmental model for β-1,6-glucan biosynthesis governed by a multi-layered regulatory network. In the ER, Kre5, Big1, Cwh41/Gls1, Rot2/Gls2, and Cne1 establish the substrate foundation, among which, Kre5 likely generates a glucosylated N-glycan acceptor for subsequent chain elongation [69,70], Big1 facilitates assembly of the synthetic machinery through a partially independent pathway [55], and the glucosi- dases together with calnexin ensure correct glycoprotein folding, their modest β-1,6-glucan reduction phenotypes (30–50%) indicating facilitatory rather than catalytic roles [58,59]; thus, the ER serves as a substrate preparation gateway, generating a primed acceptor for transport to the Golgi. In the Golgi, Kre6 and Skn1, type II transmembrane proteins with glycoside hydrolase fa

  generalfuture work
  Keywords: glucan cell wall multi biosynthesis fungal network integrity golgi chitin chain among structural protein evidence
• New Vision of Cell Walls in Aspergillus fumigatus from Solid-State NMR Spectroscopy (2024) · Journal of Fungi · cited 18× · doi

  The application of solid-state NMR techniques, coupled with biochemical findings, has enhanced our understanding of the physical characteristics of seven primary polysac- charides and their unaltered arrangement within native A. fumigatus cell walls. Chitin imparts rigidity to the cell wall, allowing molecules associated with it to attain partial rigidity, including α-1,3-glucan and β-glucans [26]. β- and α-glucans constitute the flexible matrix within the cell wall, with β-glucans facilitating water binding in A. fumigatus mycelia and α-glucans regulating water activity in A. fumigatus conidial cell walls [10,26,27]. The structural diversity of β-glucans, including linear β-1,3-glucan, terminal β-1,3/1,4-glucan, J. Fungi 2024, 10, 219 8 of 12 and branched β-1,3/1,6-glucan, is vital for maintaining molecular complexity and matrix formation. The latter two forms of glucans persist even in the absence of β-1,3-glucan; this is due to caspofungin treatment and interactions with other polysaccharides (α-1,3-glucan and chitin) to reinforce cell wall stability [47]. Although the level of chitin deacetylation is typically low in A. fumigatus mycelial and conidia samples [10,27], chitosan content may increase in response to stress [28,47]. Galactomannan is covalently linked to β-1,3-glucans, which are sometimes further cross-linked to chitin [9], but are the most mobile components within this polysaccharide complex [10]. Instead, it predominantly protrudes into the outer layer, along with galactomannan, in order to bolster the structural proteins within the cell wall [10]. Galactomannan plays a crucial role in preserving the charge of the cell wall surface, particularly for the mycelial cell wall and germinating conidia [10,27]. It also acts as a masking molecule, concealing β-glucans that are embedded deeper within the structure. Certain structural motifs, such as α-1,4-glucose residues found in α-glucan (a minor component) [6] and melanin, the aromatic-rich pigment closely associated with A. fumigatus cell walls, have not yet been evaluated using solid-state NMR (ssNMR). In addition, the galactomannan in A. fumigatus possesses a complex chemical structure, featuring a linear backbone composed of α-1,2-linked mannotetraose repeating units, bridged via α-1,6-linkage [64,65]. Some of the α-1,2-linked mannose residues of the backbone are further branched at C-6 and C-3 positions by galactofuran sidechains consisting of several (on average, 4 to 5) β-1,5-galactofuranose units [65]. This structural elucidation was achieved via the solution NMR analysis of extracted galactomannan. Solid-state NMR has only resolved signals from the predominant structural units, such as the repeating β-1,5-galactofuranoses in the side chains and the α-1,2-linked and 1,6-linked mannoses in the backbone, while the branching sites remain unresolved. The functional role of the galactofuranose sidechains of galactomannan remains unclear as well. To elucidate the functional princi

  generalfuture work
  Keywords: cell glucans glucan fumigatus wall galactomannan linked within structural chitin solid state walls backbone units
• A bipartite glucan synthase-remodeler module organizes branched glucan assembly in the fungal cell wall (2026) · Nature Communications · doi

  The paper identifies a gap in the understanding of how branches are added to the linear β-1,3-glucan backbones. It also highlights the lack of knowledge on the spatial and functional organization of enzymes involved in cell wall construction. The authors note that no specific enzyme has been shown to catalyze the remodeling step of adding branches to the β-1,3-glucan polymer.

  generalstated research gapevidence 5/5
  Keywords: paper identifies gap understanding branches added linear glucan

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