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 mocs2l, the sulfur is then transferred to precursor Z to form molybdopterin (By similarity). Cofactor biosynthesis; molybdopterin biosynthesis. Heterotetramer; composed of 2 small (mocs2s) and 2 large (mocs2l) 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. Belongs to the MoaD family. MOCS2A subfamily.