title: Bovine NAD+-dependent isocitrate dehydrogenase: alternative splicing and tissue-dependent expression of subunit 1.
abstract: NAD+-dependent isocitrate dehydrogenase (IDH), a key regulatory enzyme in the Krebs cycle, is a multi-tetrameric enzyme. At least three of the subunits in the core tetramer of mammals are unique gene products. Subunits 1/beta and 2/gamma are considered to be regulatory, while subunits 3,4/alpha, comprising half the tetramer, are catalytic. The full sequence was obtained for the major subunit 1 cDNA in bovine heart, IDH 1-A. A second cDNA, rare in heart, was also identified (IDH 1-B). Differences in the two were confined to the 3'-region, suggesting alternative splicing. Screening of brain, kidney, and liver RNA showed the presence of IDH 1-A and 1-B and a third major species, IDH 1-C. Amplification of bovine genomic DNA by PCR across the regions of difference produced a single product. Comparison of the genomic and mRNA sequences showed that IDH 1-A resulted from splicing of exon W to exon Y, eliminating intron w, exon X, and intron x. IDH 1-B was formed by splice junctions between exon W, exon X, and exon Y. IDH 1-C resulted from splicing of exon W to exon X and subsequent retention of intron x. The 2 proteins predicted from these 3 mRNAs are identical over their first 357 residues. Protein IDH 1-A, resulting from a termination codon within exon Y, contains an additional 26 residues. Proteins IDH 1-B and 1-C derive from a common termination codon within exon X and contain an additional 28 residues. The two C-terminal regions differ notably in the number and nature of charged residues, resulting in proteins with a charge difference of 3.2 at pH 7.0. Subunit 1 sequences previously reported from other species grouped with one or the other of the bovine proteins. No evidence was found for alternative splicing in subunit 3,4/alpha. The results of the present study, together with recent work on the 2/gamma subunit [Brenner,V., Nyakatura, G., Rosenthal, A., and Platzer, M. (1998) Genomics 44, 8], indicate that the regulatory subunits of the enzyme, but not the catalytic, possess alternatively spliced forms varying in C-terminal properties with tissue-specific expression. The finding is suggestive of a mechanism for modulation of allosteric regulation tailored to the needs of different tissues.
authors.affiliation: Department of Biochemistry, State University of New York Downstate Medical Center, 450 Clarkson Avenue, Brooklyn, New York 11203, USA.
authors.forename: C
authors.initials: C
authors.lastname: Weiss
authors.forename: Y
authors.initials: Y
authors.lastname: Zeng
authors.forename: J
authors.initials: J
authors.lastname: Huang
authors.forename: M B
authors.initials: MB
authors.lastname: Sobocka
authors.forename: J I
authors.initials: JI
authors.lastname: Rushbrook
mesh_terms: D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001921:Brain / Q000378:metabolism; D002417:Cattle; D005786:Gene Expression Regulation; D006801:Humans; D007521:Isocitrate Dehydrogenase / Q000096:biosynthesis* / Q000235:genetics*; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008252:Macaca fascicularis; D008969:Molecular Sequence Data; D009206:Myocardium / Q000378:metabolism; D009243:NAD / Q000378:metabolism*; D009928:Organ Specificity / Q000235:genetics; D012333:RNA, Messenger / Q000096:biosynthesis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA
chemical_list: D012333:RNA, Messenger; D009243:NAD; D007521:Isocitrate Dehydrogenase