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 UBA4. 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 UBA4, then thiocarboxylated (-COSH) via the rhodanese domain of UBA4. Belongs to the MoaD family. MOCS2A subfamily.