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. Expressed in both embryo and endosperm during seed maturation. 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. Precocious germination and early seedling lethality; probably due to molybdenum cofactor (MoCo) defects, a cofactor required for abscisic acid (ABA) biosynthesis. Belongs to the MoaD family. MOCS2A subfamily.