Gene structures of three vertebrate adenylate kinase isozymes

Adenylate kinase is an ubiquitous enzyme which contributes to homeostasis of adenine nucleotide composition in the cell. In vertebrates, three isozymes (AK1, AK2, and AK3) are characterized which have distinct distribution in tissues as well as subcellular compartments. The genetic backgrounds of these adenylate kinase isozymes were analyzed. cDNA clones for AK1 were isolated from poly(A)+RNA of chicken skeletal muscle. The results of mRNA analysis in various tissues using the AK1 cDNA indicated that the AK1 gene expression is regulated both tissue-specifically and developmentally at the transcriptional level. The AK1 gene was cloned from chicken and human DNA and characterized. Both genes were split into seven exons. The intron positions in both genes coincided. cDNA clones for AK2 isolated from bovine liver poly(A)+RNA contained two types. One type (AK2A) encoded the same amino acid sequence as that reported for bovine heart AK2. The other type (AK2B) encoded the same sequence as AK2 except for the COOH terminus. The mRNA species corresponding to the two cDNA clones were identified in bovine liver and heart. Both the cDNA sequences were found to direct the active adenylate kinase synthesis in E. coli. The AK2 gene was cloned and characterized. It consisted of seven exons and six introns. From genomic structure analysis, the two cDNA species were shown to be derived from a single gene by the alternative splicing mechanism. Three types of cDNA clones for AK3 were isolated from bovine liver poly(A)+RNA, which contained the common AK3-coding region and different 3' portions. No NH2-terminal presequence of mitochondrial targeting was identified in AK3 from the sequencing and expression analyses of the cDNA. Upon expression of the cDNA sequence in E. coli, AK3 protein was recovered in the periplasmic space of the bacteria, indicating that AK3 without presequence was exported through the inner bacterial membrane as it is imported through the mitochondrial membranes. Internal targeting signals may be responsible for the translocation process. The AK3 gene was cloned and partially characterized. It is split into at least five exons. The comparisons of amino acid sequences and genomic structure of three isozymes revealed that a segment corresponding to either exon 5 of the AK2 gene or a part of exon 3 of the AK3 gene is missing in the AK1 gene. Phylogenetic analysis suggested that AK1, a shorter molecule, would have been separated from a longer molecule very early in evolution of adenylate kinase.(ABSTRACT TRUNCATED AT 400 WORDS)