Structure of the Escherichia coli ThiS-ThIF complex, a key component of the sulfur transfer system in thiamin biosynthesis

We have determined the crystal structure of the Escherichia coli ThiS-ThiF protein complex at 2.0 angstrom resolution. ThiS and ThiF are bacterial proteins involved in the synthesis of the thiazole moiety of thiamin. ThiF catalyzes the adenylation of the carboxy terminus of ThiS and the subsequent displacement of AMP catalyzed by Thil-persulfide to give a ThiS-ThiI acyl disulfide. Disulfide interchange, involving Cys184 on ThiF, then generates the ThiS-ThiF acyl disulfide, which functions as the sulfur donor for thiazole formation. ThiS is a small 7.2 kDa protein that structurally resembles ubiquitin and the molybdopterin biosynthetic protein MoaD. ThiF is a 27 kDa protein with distinct sequence and structural similarity to the ubiquitin activating enzyme E1 and the molybdopterin biosynthetic protein MoeB. The ThiF-ThiS structure clarifies the mechanism of the sulfur transfer chemistry involved in thiazole biosynthesis.