Creatine kinase (CK; EC 18.104.22.168) plays an important role in energy metabolism in brain and muscle. Expression of CK isoenzymes is regulated during development and is tissue specific. To define the structures of canine CK isoenzymes and to elucidate the mechanism of regulation in their expression, CK cDNA clones from dog myocardium were isolated. Myocardial CK mRNA is predicted to encode a protein of 381 amino acids. The nontranslated regions of the mRNA comprise at least 38 bases at the 5' end and exactly 345 bases before the poly(A) tail. Partial protein sequences of dog muscle (M) CK and brain (B) CK subunits were determined and compared with the derived amino acid sequence of the myocardial enzyme and of M CK subunits of other species. The M CK subunits from different species share a very high degree (83-96%) of sequence identity. Dog M and B subunits share extensive sequence identity (74%), a degree of similarity not previously suspected. Southern blot analysis suggests that a CK gene family exists. These observations imply that evolutionary changes in the M CK subunit structure are constrained by the need for preservation of functional properties other than the kinase activity. This conservation is consistent with the possibility that the M subunit plays a structural role in cardiac and skeletal muscle.
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