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. Intron retention.