Eng1 and Exg8 Are the Major β-Glucanases Secreted by the Fungal PathogenHistoplasma capsulatum.
Fungal cell walls contain β-glucan polysaccharides that stimulate immune responses when recognized by host immune cells. The fungal pathogenHistoplasma capsulatumminimizes detection of β-glucan by host cells through at least two mechanisms: concealment of β-glucans beneath α-glucans and enzymatic removal of any exposed β-glucan polysaccharides by the secreted glucanase Eng1.Histoplasmayeasts also secrete the putative glucanase Exg8, which may serve a similar role as Eng1 in removing exposed β-glucans from the yeast cell surface. Here, we characterize the enzymatic specificity of the Eng1 and Exg8 proteins and show that Exg8 is an exo-β1,3-glucanase and Eng1 is an endo-β1,3-glucanase. Together, Eng1 and Exg8 account for nearly all of the total secreted glucanase activity ofHistoplasmayeasts. Both Eng1 and Exg8 proteins are secreted through a conventional secretion signal and are modified post-translationally byO-linked glycosylation. Both glucanases have near maximal activity at temperature and pH conditions experienced during infection of host cells, supporting roles inHistoplasmapathogenesis. Exg8 has a higher specific activity than Eng1 for β1,3-glucans; yet despite this, Exg8 does not reduce detection of yeasts by the host β-glucan receptor Dectin-1. Exg8 is largely dispensable for virulencein vivo, in contrast to Eng1. These results show thatHistoplasmayeasts secrete two β1,3-glucanases and that Eng1 endoglucanase activity is the predominant factor responsible for removal of exposed cell wall β-glucans to minimize host detection ofHistoplasmayeasts.
From the Departments of Microbiology, Microbial Infection, and Immunity and.
Andrew L
AL
Garfoot
From the Departments of Microbiology, Microbial Infection, and Immunity and.
Kacey L
KL
Dearing
Chemistry and Biochemistry, Ohio State University, Columbus, Ohio 43210.
Andrew D
AD
VanSchoiack
Chemistry and Biochemistry, Ohio State University, Columbus, Ohio 43210.
Vicki H
0000-0003-0495-2538
VH
Wysocki
From the Departments of Microbiology, Microbial Infection, and Immunity and rappleye.1@osu.edu.
Chad A
0000-0001-7880-5958
CA
Rappleye
D043326:Glucan 1,3-beta-Glucosidase / Q000378:metabolism*; D004693:Glucan Endo-1,3-beta-D-Glucosidase / Q000378:metabolism*; D006658:Histoplasma / Q000201:enzymology* / Q000378:metabolism / Q000472:pathogenicity; D006660:Histoplasmosis / Q000382:microbiology*; D006801:Humans; D013379:Substrate Specificity; D047071:beta-Glucans / Q000378:metabolism
D047071:beta-Glucans; D004693:Glucan Endo-1,3-beta-D-Glucosidase; D043326:Glucan 1,3-beta-Glucosidase; C117648:beta-1,3-exoglucanase
Histoplasma; cell wall; fungi; glucanase; macrophage; microbial pathogenesis; polysaccharide; β-glucan