HOW AND WHY CERTAIN MEMBRANE PROTEINS ARE DELIVERED TO THE FUNGAL CELL WALL

The transfer of GPI-anchored substrates from the plasma membrane to the cell wall as catalysed by a unique type of glycoside hydrolases is essential for fungal life. Combined structural, computational and in-vivo analysis reveal this important step in fungal cell wall biogenesis, providing insights into the sorting mechanism at the outer leaflet and a structural base for the development of new, potentially side effect-free antimycotics.

STRUCTURAL BIOLOGY

48 ESRF

The Human LL-37(17-29) antimicrobial peptide reveals a functional supramolecular structure, Y. Engelberg (a) and M. Landau (a,b), Nat. Commun. 11, 3894

(2020); https://doi.org/10.1038/s41467- 020-17736-x. (a) Department of Biology, Technion-Israel Institute of Technology, Haifa (Israel)

(b) Centre for Structural Systems Biology (CSSB), and European Molecular Biology Laboratory (EMBL), Hamburg (Germany)

[1] E. Tayeb-Fligelman et al., Science 355, 831-833 (2017).

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

Structure-guided mutagenesis suggested the important role of self-assembly in antibacterial activity. Moreover, the LL-3717-29 fibrils display thermostability up to 80˚C and a high zeta potential compared to non-fibrillating mutants or other helical protein polymers. The findings offer a promising approach to develop durable antimicrobials as an alternative to common antibiotics for coating of medical devices, implants and stents and, potentially, as therapeutics for autoimmune disorders and as anti-cancer supportive therapy. Moreover,

controlling fibrillation might provide controlled- release medications under specific triggers.

Interestingly, LL-3717-29 shares sequence similarity and the ability to form helical fibrils with the bacterial toxic peptide PSMα3, which forms cross-α amyloid fibrils that play a role in killing human immune cells [1]. This suggests a possible molecular or structural mimicry mechanism used by the bacteria to provide immune-evasive and survival strategies. The similarity also points to potential functional building blocks across kingdoms of life in the form of densely packed amphipathic helical fibrils, complementing the exciting hypotheses about short amyloid peptides serving as prebiotic information-coding molecules.

Fig. 34: Atomic structures of human and gorilla LL-3717-29 fibrils, with view down the fibril axis, featuring the similar hexameric architecture of densely packed helices, with a nanotube running along the fibril. The two N-terminal residues, phenylalanine (human) or serine (gorilla) at position 17, and lysine at position 18, are facing the pore. These residues show a more extended conformation in the gorilla structure, leading to a more occluded nanotube.

Systemic infections by invasive fungal pathogens in immunocompromised patients are an increasing, yet underestimated and unsatisfactorily addressed threat to human health [1]. The fungal cell wall biogenesis is a promising candidate for side effect-free drug targets, as the underlying architecture and features are unique for fungi [2]. A key step in the synthesis and maturation of the cell wall is the transfer of a subset of glycosylphosphatidylinositol (GPI) anchored proteins from the plasma membrane into the cell wall (Figure 35a). In baker s yeast, the underlying

transglycosylation has long been suggested to be catalysed by two homologous glycoside hydrolases of the GH76 family, Dfg5 (defective in filamentous growth) and Dcw1 (defective cell wall) [3]. As they are related to bacterial GH76 homologues, an α1,6-mannanase function of Dfg5 enzymes in filamentous fungi has also been suggested [4,5].

The crystal structure of Dfg5 from the filamentous fungus Chaetomium thermophilum shows that the overall fold and the active site DD-motif is conserved among GH76 enzymes,