CHROMOSOMAL LOCALIZATION AND CATALYTIC PROPERTIES OF THE RECOMBINANT ALPHA-SUBUNIT OF HUMAN LYMPHOCYTE METHIONINE ADENOSYLTRANSFERASE

Human lymphocyte methionine adenosyltransferase (HuLy MAT) consists of heterologous subunits alpha and beta. The cDNA sequence of the alpha subunit of HuLy MAT from Jurkat leukemic T cells was identical to that of the human kidney alpha subunit and highly homologous to the sequen ce of the extrahepatic MAT from other sources, The 3'-untranslated seq uence was found to be highly conserved, suggesting that it may be impo rtant in regulating the expression of MAT. The extrahepatic alpha subu nit of MAT was found to be expressed also in human liver, and no diffe rences were found in the sequence of the alpha 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 clo nes harboring the human extrahepatic alpha subunit gene and to localiz e it to the centromere on chromosome arm 2p, an area that corresponds to band 2p11.2. Expression of the alpha subunit cDNA in Escherichia co li yielded two peptides with the immunoreactivity and mobilities of au thentic alpha/alpha' subunits from HuLy, The K-m of the recombinant al pha subunit was 80 mu M, which is 20-fold higher than found for the (a lpha alpha')(x) beta(y) holoenzyme purified from leukemic lymphocytes and 4-10-fold higher than found for the normal lymphocyte enzyme. The data suggest that the alpha/alpha' subunits mediate the enzyme catalyt ic activity and that the beta subunit may be a regulatory subunit of e xtrahepatic MAT.