9.B.119 The Glycan Synthase, Fks1 (Fks1) Family

Fks1, with orthologs in nearly all fungi as well as plants and many protists, plays a central role in fungal cell wall formation as the putative catalytic component of β-1,3-glucan synthase. It is also the target for an important new antifungal group, the echinocandins, as evidenced by the localization of resistance-conferring mutations to Fks1 hotspots 1, 2 and 3 (residues 635-649, 1354-1361, and 690-700, respectively). Johnson and Edlind (2012) used HA-Suc2-His4C fusions to C-terminally truncated Saccharomyces cerevisiae Fks1 to experimentally define its topology, and site-directed mutagenesis to test function of selected residues. Of the 15 to 18 TMSs predicted for Fks1 from evolutionarily diverse fungi, 13 were experimentally confirmed. The N-terminus (residues 1-445) is cytosolic and the C-terminus (residues 1823-1876) external; both are essential to Fks1 function. The cytosolic central domain (residues 715-1294) includes homology to glycosyltransferases, and residues potentially involved in substrate UDP-glucose binding and catalysis are essential. All three hotspots are external, with hotspot 1 adjacent to and hotspot 3 largely embedded within the outer leaflet of the membrane. This topology suggests a model in which echinocandins interact through their lipid tails with hotspot 3 and through their cyclic peptides with hotspots 1 and 2 (Johnson and Edlind, 2012). A transport function is possible but not identified.


 

References:

Johnson ME. and Edlind TD. (2012). Topological and mutational analysis of Saccharomyces cerevisiae Fks1. Eukaryot Cell. 11(7):952-60.
Yang, Y.M., X. Fu, F.J. Cui, L. Sun, X.Y. Zan, and W.J. Sun. (2023). Biochemical and structural characterization of a glucan synthase GFGLS2 from edible fungus Grifola frondosa to synthesize β-1, 3-glucan. Biotechnol Biofuels Bioprod 16: 163.
Zhao, C.R., Z.L. You, D.D. Chen, J. Hang, Z.B. Wang, M. Ji, L.X. Wang, P. Zhao, J. Qiao, C.H. Yun, and L. Bai. (2023). Structure of a fungal 1,3-β-glucan synthase. Sci Adv 9: eadh7820.
Examples:

TC#NameOrganismal TypeExample
9.B.119.1.1

1,3-β-glucan synthase component, Fks1 of 1876 aas and about 11 TMSs in a 6 + 5 + 5 TMS arrangement.  1,3-beta-glucan serves as the primary component of the fungal cell wall; the synthase is located in the plasma membrane where it is a molecular target for antifungal drugs such as echinocandins and the triterpenoid ibrexafungerp. Zhao et al. 2023 presented the cryo-EM structure of Fks1 at 2.47 Å resolution. The structure revealed a central catalytic region adopting a cellulose synthase fold with a cytosolic conserved GT-A-type glycosyltransferase domain and a closed transmembrane channel responsible for glucan export. Two extracellular disulfide bonds are crucial for Fks1 enzymatic activity. 

Yeast

Fks1 of Saccharomyces cerevisiae (P38631)

 
9.B.119.1.2

Putative 1,3-β-D-glucan synthase

Plants

Kks1 homologue of Arabidopsis thaliana (Q3B724)

 
9.B.119.1.3

Callose synthase, CalS

Plants

CalS of Arabidopsis thaliana (Q9LUD7)

 
9.B.119.1.4

Glycosoyl transferase

Algae

Glycosyl transferase of Chlamydomonas reinhardtii (A8JK32)

 
9.B.119.1.5

Putative glucan synthase 

Plants

Putative glucan synthase of Arabidopsis thaliana (Q8LPI6)

 
9.B.119.1.6

1,3-beta-D-glucan synthase, GFGLS2, of 1713 aas and 10 TMSs in a 4 (residues 390 - 510) + 4 (residues 560 - 700) + 4 (residues 1190 - 1360) + 4 (residues 1440 - 1570) + 1 (resdue 1630) + 1 (residue 1690).  The biochemical, 3D structural characterization and potential catalytic mechanism of this membrane-bound beta-1, 3-glucan synthase, GFGLS2 from cultured mycelia of G. frondosa was investigated and provides a reasonably full picture of beta-1, 3-glucan synthesis in fungi (Yang et al. 2023).

GFGLS2 of Grifola frondosa