Regulatory subunit of S-adenosylmethionine synthetase 2, an enzyme that catalyzes the formation of S-adenosylmethionine from methionine and ATP. Regulates MAT2A catalytic activity by changing its kinetic properties, increasing its affinity for L-methionine. Can bind NADP (in vitro). Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis; S-adenosyl-L-methionine from L-methionine: step 1/1. Heterotrimer; composed of a catalytic mat2a homodimer that binds one regulatory mat2b chain. Heterohexamer; composed of a central, catalytic mat2a homotetramer flanked on either side by a regulatory mat2b chain. NADP binding increases the affinity for mat2a. Belongs to the dTDP-4-dehydrorhamnose reductase family. MAT2B subfamily.