Acts as a sulfur carrier required for molybdopterin biosynthesis. Component of the molybdopterin synthase complex that catalyzes the conversion of precursor Z into molybdopterin by mediating the incorporation of 2 sulfur atoms into precursor Z to generate a dithiolene group. In the complex, serves as sulfur donor by being thiocarboxylated (-COSH) at its C-terminus by MOCS3. After interaction with Mocs2B, the sulfur is then transferred to precursor Z to form molybdopterin. Cofactor biosynthesis; molybdopterin biosynthesis. Heterotetramer; composed of 2 small (Mocs2A) and 2 large (Mocs2B) subunits. C-terminal thiocarboxylation occurs in 2 steps, it is first acyl-adenylated (-COAMP) via the hesA/moeB/thiF part of MOCS3, then thiocarboxylated (-COSH) via the rhodanese domain of MOCS3. This protein is produced by a bicistronic gene which also produces the large subunit (Mocs2B). Belongs to the MoaD family. MOCS2A subfamily.