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 (PubMed:10644686, PubMed:23189196, PubMed:25075345). Can bind NADP (in vitro) (PubMed:23189196, PubMed:23425511).
