alpha-1,3-Glucan is a homopolymer composed of D-glucose (Glc) and it is an extracellular polysaccharide found in dental plaque due to Streptococcus species. alpha-1,3-Glucanase from Streptomyces thermodiastaticus strain HF3-3 (Agl-ST) has been identified as a thermostable alpha-1,3-glucanase, which is classified into glycoside hydrolase family 87 (GH87) and specifically hydrolyzes alpha-1,3-glucan with an endo-action. The enzyme has a potential to inhibit the production of dental plaque and to be used for biotechnological applications. Here we show the structure of the catalytic unit of Agl-ST determined at 1.16 angstrom resolution using X-ray crystallography. The catalytic unit is composed of two modules, a beta-sandwich fold module, and a right-handed beta-helix fold module, which resembles other structural characterized GH87 enzymes from Bacillus circulans str. KA-304 and Paenibacillus glycanilyticus str. FH11, with moderate sequence identities between each other (approximately 27% between the catalytic units). However, Agl-ST is smaller in size and more thermally stable than the others. A disulfide bond that anchors the C-terminal coil of the beta-helix fold, which is expected to contribute to thermal stability only exists in the catalytic unit of Agl-ST. (C) 2020 Elsevier Inc. All rights reserved.