Synthase catalytic subunit of the trehalose synthase complex that catalyzes the production of trehalose from glucose-6-phosphate and UDP-alpha-D-glucose in a two step process (PubMed:8021171). The disaccharide trehalose serves as a storage carbohydrate that is mobilized during nutrient stress and spore germination (PubMed:8021171, PubMed:9729425). Together with ntp1, regulates the level of trehalose as a protectant for cell integrity during thermal and osmotic stress (PubMed:9495778, PubMed:8021171). D-glucose 6-phosphate + UDP-alpha-D-glucose = alpha,alpha-trehalose 6-phosphate + H(+) + UDP Carbohydrate biosynthesis. Homomer (PubMed:12153582). Component of the trehalose synthase complex that contains at least tps1, ntp1 and tpp1 (PubMed:12153582). Interacts with tpp1 (PubMed:12153582). Interacts with ntp1; the interaction is independent of stress conditions (PubMed:12153582, PubMed:12943532, PubMed:15965643). By thermal stress. Abolishes cytoplasmic neutral trehalase activation during thermal stress; activation during osmotic stress is normal (PubMed:9729425). Decreases cellular trehalose 6-phosphate level in stationary phase and during thermal stress (PubMed:8021171). Sensitive to heat shock and osmotic stress (PubMed:9495778). Belongs to the glycosyltransferase 20 family.