DIFFERENTIAL LOCALIZATION OF THE MESSENGER-RNA OF THE M-ISOFORMS AND B-ISOFORMS OF CREATINE-KINASE IN MYOBLASTS

Creatine kinase (CK) plays an important role in buffering ATP and ADP levels in tissues which have intermittently high and fluctuating energ y demands, such as skeletal muscle. This buffering function has a spat ial, as well as a temporal aspect, which is dependent on the localizat ion of different enzyme isoforms within the cell. We show here, by in situ hybridization, that the mRNAs for the cytoplasmic isoforms of CK are differentially localized in a mouse myoblast cell line (C2C12). Th e mRNA for the M form is localized at the cell periphery, while that f or the B form is localized in the perinuclear region. Deletion of segm ents of the 3' untranslated regions of these mRNAs or swapping of thes e segments between the mRNAs for the two isoforms demonstrated that lo calization signals lie within these regions. Localization appears to b e tissue-specific, since both the M and B mRNAs were distributed unifo rmly over the cytoplasm in a non-muscle cell line. These results, in c onjunction with other studies which have shown that mRNA localization can lead to co-localization of the encoded protein, suggest that the l ocalization of the mRNAs for the cytoplasmic isoforms of CK may be inv olved in the localization of the enzymes themselves.