Chromosomal Localization and Catalytic Properties of the Recombinant α Subunit of Human Lymphocyte Methionine Adenosyltransferase (∗)

Human lymphocyte methionine adenosyltransferase (HuLy MAT) consists of heterologous subunits α and β. The cDNA sequence of the α subunit of HuLy MAT from Jurkat leukemic T cells was identical to that of the human kidney α subunit and highly homologous to the sequence of the extrahepatic MAT from other sources. The 3′-untranslated sequence was found to be highly conserved, suggesting that it may be important in regulating the expression of MAT. The extrahepatic α subunit of MAT was found to be expressed also in human liver, and no differences were found in the sequence of the α subunit from normal and malignant T cells. The sequence of two unspliced introns found in the cDNA clones from the Jurkat library enabled us to isolate genomic clones harboring the human extrahepatic α subunit gene and to localize it to the centromere on chromosome arm 2p, an area that corresponds to band 2p11.2. Expression of the α subunit cDNA in Escherichia coli yielded two peptides with the immunoreactivity and mobilities of authentic α/α‘ subunits from HuLy. The Km of the recombinant α subunit was 80 μM, which is 20-fold higher than found for the (αα′)xβy holoenzyme purified from leukemic lymphocytes and 4-10-fold higher than found for the normal lymphocyte enzyme. The data suggest that the α/α‘ subunits mediate the enzyme catalytic activity and that the β subunit may be a regulatory subunit of extrahepatic MAT.