Glycoside hydrolase from the GH76 family indicates that marine Salegentibacter sp. Hel6 consumes alpha-mannan from fungi.

Solanki, Vipul; Krüger, Karen; Crawford, Conor J.; Pardo-Vargas, Alonso; Danglad-Flores, José; Hoang, Kim Le Mai; Klassen, Leeann; Abbott, D. Wade; Seeberger, Peter H.; Amann, Rudolf I.; Teeling, Hanno; Hehemann, Jan-Hendrik

Microbial glycan degradation is essential to global carbon cycling. The marine bacterium Salegentibacter sp. Hel6 (Bacteroidota) isolated from seawater off Helgoland island (North Sea) contains an α-mannan inducible gene cluster with a GH76 family endo-α-1,6-mannanase (ShGH76). This cluster is related to genetic loci employed by human gut bacteria to digest fungal α-mannan. Metagenomes from the Hel6 isolation site revealed increasing GH76 gene frequencies in free-living bacteria during microalgae blooms, suggesting degradation of α-1,6-mannans from fungi. Recombinant ShGH76 protein activity assays with yeast α-mannan and synthetic oligomannans showed endo-α-1,6-mannanase activity. Resolved structures of apo-ShGH76 (2.0 Å) and of mutants co-crystalized with fungal mannan-mimicking α-1,6-mannotetrose (1.90 Å) and α-1,6-mannotriose (1.47 Å) retained the canonical (α/α)6 fold, despite low identities with sequences of known GH76 structures (GH76s from gut bacteria: <27%). The apo-form active site differed from those known from gut bacteria, and co-crystallizations revealed a kinked oligomannan conformation. Co-crystallizations also revealed precise molecular-scale interactions of ShGH76 with fungal mannan-mimicking oligomannans, indicating adaptation to this particular type of substrate. Our data hence suggest presence of yet unknown fungal α-1,6-mannans in marine ecosystems, in particular during microalgal blooms.

ISME Journal: Multidisciplinary Journal of Microbial Ecology, 2022, Vol 16, Issue 7, p1818

1751-7362

Academic Journal

10.1038/s41396-022-01223-w