Methionine adenosyltransferase (MAT) catalyzes the synthesis of S-adenosylmethionine (AdoMet). The mammalian MAT II isozyme consists of catalytic alpha(2) and regulatory beta subunits. The aim of this study was to investigate the interaction and kinetic behavior of the human MAT II subunit proteins in mammalian cells. COS-1 cells were transiently transfected with pTargeT vector harboring full-length cDNA that encodes for the MAT II alpha(2) or beta subunits. Expression of the His-tagged recombinant alpha(2) (ralpha(2)) subunit in COS-1 cells markedly increased MAT II activity and resulted in a shift in the K(m) for L-methionine (L-Met) from 15 microM (endogenous MAT II) to 75 microM (ralpha(2)), and with the apparent existence of two kinetic forms of MAT in the transfected COS-1 cell extracts. By contrast, expression of the recombinant beta (rbeta) subunit had no effect on the K(m) for L-Met of the endogenous MAT II, while it did cause an increase in both the V(max) and the specific activity of endogenous MAT. Co-expression of both ralpha(2) and rbeta subunits resulted in a significant increase of MAT specific activity with the appearance of a single kinetic form of MAT (K(m) = 20 microM). The recombinant MAT II alpha(2) and rbeta subunit associated spontaneously either in cell-free system or in COS-1 cells co-expressing both subunits. Analysis of nickel-agarose-purified His-tagged ralpha(2) subunit from COS-1 cell extracts showed that the beta subunit co-purified with the alpha(2) subunit. Furthermore, the alpha(2) and beta subunits co-migrated in native polyacrylamide gels. Together, the data provide evidence for alpha(2) and beta MAT subunit association. In addition, the beta subunit regulated MAT II activity by reducing its K(m) for L-Met and by rendering the enzyme more susceptible to feedback inhibition by AdoMet. We believe that the previously described differential expression of MAT II beta subunit may be an important mechanism by which MAT activity can be modulated to provide different levels of AdoMet that may be required at different stages of cell growth and differentiation.