Structural characterization and tissue-specific expression of the mRNAs encoding isoenzymes from two rat mitochondrial creatine kinase genes.

Creatine kinase (CK; EC 2.7.3.2) isoenzymes play prominent roles in energy transduction. Mitochondrial CK (MtCK) reversibly catalyzes the transfer of high energy phosphate to creatine and exists, in the human, as two isoenzymes encoded by separate genes. We report here the cDNA sequences of the two isoenzymes of MtCK in the rat. Rat sarcomeric MtCK has 87% nucleotide identity in the 1257 bp coding region and 82% in the 154 bp 3' untranslated region as compared with human sarcomeric MtCK. Rat ubiquitous MtCK has 92% nucleotide identity over the 1254 bp coding region with human ubiquitous MtCK and 81% identity of the 148 by 3' untranslated region. Nucleotide identity between the rat sarcomeric and ubiquitous MtCK coding regions is 70%, with no conservation of their 3' untranslated regions. Thus, MtCK sequence is conserved in a tissue-specific, rather than species-specific, manner. Conservation of the 3' untranslated regions is highly unusual and suggests a regulatory function for this region. The NH2-terminal transit peptide sequences share 82% amino acid homology between rat and human sarcomeric MtCKs and 92% homology between rat and human ubiquitous MtCKs, but have only 41% homology to each other. This tissue-specific conservation of the transit peptides suggests receptor specificity in mitochondrial uptake. Rat sarcomeric MtCK mRNA is expressed only in skeletal muscle and heart, but rat ubiquitous MtCK mRNA is expressed in many tissues, with highest levels in brain, gut and kidney. Ubiquitous MtCK mRNA levels are dramatically regulated in uterus and placenta during pregnancy. Coexpression of sarcomeric and ubiquitous MtCK with their cytosolic counterparts, MCK and BCK, respectively, supports the creatine phosphate shuttle hypothesis and suggests that expression of these genes is coordinately regulated.