Expression and Functional Interaction of the Catalytic and Regulatory Subunits of Human Methionine Adenosyltransferase in Mammalian Cells

Abstract

. The recombinant MAT II ␣ 2 and r␤ subunit associated spontaneously either in cell-free system or in COS-1 cells coexpressing both subunits. Analysis of nickel-agarosepurified His-tagged r␣ 2 subunit from COS-1 cell extracts showed that the ␤ subunit co-purified with the ␣ 2 subunit. Furthermore, the ␣ 2 and ␤ subunits co-migrated in native polyacrylamide gels. Together, the data provide evidence for ␣ 2 and ␤ MAT subunit association. In addition, the ␤ 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 ␤ 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.

Regulation of the Human MAT2B Gene Encoding the Regulatory β Subunit of Methionine Adenosyltransferase, MAT II

“…In contrast to MAT ␣ subunits, which are highly conserved throughout evolution, the ␤ subunit of MAT II seems to be only present in the mammalian species (15,19). Recently, we cloned and characterized the human MAT II ␤ subunit (26,31), found that it has no catalytic activity, and confirmed that it acts as a regulatory subunit for the enzyme. When ␤ associates with the ␣ subunit it alters its kinetic properties and renders MAT II more susceptible to product inhibition by AdoMet (26,31).…”

“…Recently, we cloned and characterized the human MAT II ␤ subunit (26,31), found that it has no catalytic activity, and confirmed that it acts as a regulatory subunit for the enzyme. When ␤ associates with the ␣ subunit it alters its kinetic properties and renders MAT II more susceptible to product inhibition by AdoMet (26,31). Interestingly, the human ␤ subunit can also interact with the ␣1 subunit of MAT I/III and the Escherichia coli ␣ MAT subunit and alter their kinetic properties as well (26,31).…”

“…The ␣2/␤ hetero-oligomeric composition of MAT II was also determined in bovine brain, Ehrlich ascites tumor, and calf thymus (15,19). The ␣2 subunit is responsible for the enzyme catalytic activity and is post-translationally modified to generate ␣2Ј (15,(22)(23)(24)(25)(26). Recently, Halim et al (27) reported the characterization and regulation of the human MAT2A gene and found it to be remarkably similar to that of the rat and mouse MAT2A genes (28, 29) as well as to the human MAT1A gene (30), which encodes the liver-specific ␣1 subunit.…”

“…When ␤ associates with the ␣ subunit it alters its kinetic properties and renders MAT II more susceptible to product inhibition by AdoMet (26,31). Interestingly, the human ␤ subunit can also interact with the ␣1 subunit of MAT I/III and the Escherichia coli ␣ MAT subunit and alter their kinetic properties as well (26,31).…”

“…When ␤ associates with the ␣ subunit it alters its kinetic properties and renders MAT II more susceptible to product inhibition by AdoMet (26,31). Interestingly, the human ␤ subunit can also interact with the ␣1 subunit of MAT I/III and the Escherichia coli ␣ MAT subunit and alter their kinetic properties as well (26,31).The expression of the MAT II ␣2, ␣2Ј, and ␤ subunits varies considerably in different tissues. The ␣2, ␣2Ј, and ␤ subunits are constitutively expressed at high levels in leukemic cells and at low levels in normal resting T cells.…”