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. 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.

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MAT II, the extrahepatic form of methionine adenosyltransferase (MAT), consists of catalytic ␣ 2 /␣ 2 subunits and a noncatalytic ␤ subunit, believed to have a regulatory function. The full-length cDNA that encodes the ␤ subunit of human MAT II was cloned and found to encode for a 334-amino acid protein with a calculated molecular weight of 37,552. Analysis of sequence homology showed similarity with bacterial enzymes that catalyze the reduction of TDP-linked sugars. The ␤ subunit cDNA was cloned into the pQE-30 expression vector, and the recombinant His tagged protein, which was expressed in Escherichia coli, was recognized by antibodies to the human MAT II, to synthetic peptides copying the sequence of native ␤ subunit protein, and to the r␤ protein. There is no cross-reactivity between the MAT II ␣ 2 or ␤ subunits. None of the anti-␤ subunit antibodies reacted with protein extracts of E. coli host cells, suggesting that these bacteria have no ␤ subunit protein. Interestingly, the r␤ subunit associated with E. coli as well as human MAT ␣ subunits. This association changed the kinetic properties of both enzymes and lowered the K m of MAT for L-methionine. Together, the data show that we have cloned and expressed the human MAT II ␤ subunit and confirmed its long suspected regulatory function. This knowledge affords a molecular means by which MAT activity and consequently the levels of AdoMet may be modulated in mammalian cells.Methionine adenosyltransferase (MAT; S-adenosyl-L-methionine synthetase, EC 2.5.1.6) 1 is an essential enzyme that catalyzes the synthesis of S-adenosylmethionine (AdoMet) from L-methionine (L-Met) and ATP (1, 2). AdoMet is the major methyl group donor, participating in the methylation of proteins, DNA, RNA, phospholipids, and other small molecules (reviewed in Refs. 3-5). In addition, AdoMet is the ultimate source of the propylamine moiety used in polyamine biosynthesis, and it serves as co-factor for other key enzymes in the one-carbon metabolism pathway (3-5). MAT is present in all living species, including thermophilic archaebacteria, plants, yeast, and mammals (reviewed in Refs. 4 and 6 -8). Interestingly, most species have more than one MAT isozyme (6).In mammals, it is now established that there are at least two MAT isozymes (9 -12). MAT I/III is expressed only in liver and has a catalytic subunit designated ␣ 1 that is encoded by the MAT1A gene (8, 9, 13-16). MAT I and MAT III represent different oligomeric forms of the ␣ 1 subunit -MAT III is a dimer, and MAT I is a tetramer of the ␣ 1 subunit (9, 17-19). MAT I and MAT III differ considerably in their physical, kinetic, and regulatory properties (8,9,20). The MAT II isozyme is expressed in all tissues, including the liver, and has been studied in many tissues including erythrocytes, lymphocytes, brain, kidney, testis, and fetal liver (11, 20 -27).We have been characterizing the human MAT II from human lymphocytes (22, 28 -31) 2 and were able to show that the form present in activated lymphocytes consists of distinct subunits (22...