Thermoplasma trehalase Tvn1315 is predicted to be composed of a beta-sandwich domain (BD) and a catalytic domain (CD) based on the structure of the bacterial GH15 family glucoamylase (GA). Tvn1315 as well as Tvn1315 (Delta 5), in which the 5 N-terminal amino acids are deleted, could be expressed in Escherichia coli as active enzymes, but deletion of 10 residues (Delta 10) led to inclusion body formation. To further investigate the role of the N-terminal region of BD, we constructed five mutants of Delta 5, in which each of the 5th to 10th residues of the N-terminus of Tvn1315 was mutated to Ala. Every mutant protein could be recovered in soluble form, but only a small fraction of the Y9A mutant was recovered in the soluble fraction. The Y9A mutant recovered in soluble form had similar specific activity to the other proteins. Subsequent mutation analysis at the 9th position of Tvn1315 in Delta 5 revealed that aromatic as well as bulky hydrophobic residues could function properly, but residues with hydroxy groups impaired the solubility. Similar results were obtained with mutants based on untruncated Tvn1315. When the predicted BD, Delta 5BD, Delta 10BD, and BD mutants were expressed, the Delta 10BD protein formed inclusion bodies, and the BD mutants behaved similarly to the Delta 5 and full-length enzyme mutants. These results suggest that the hydrophobic region is involved in the solubilization of BD during the folding process. Taken together, these results indicate that the solubility of CD depends on BD folding.