Fibre type specific expression of skeletal and cardiac muscle myotonin kinase

Myotonin kinase (MtPk) is the protein coded by the Myotonic Distrophy (DM) gene. The primary structure of MtPK has been deduced from the DNA sequence and indicates that the gene product is a protein kinase, most likely a Ser/Thr kinase. The presence of a polymorphic CTG-repeat expansion in pathological genes has been indicated as the molecular lesion leading to the DM phenotype. The bulk of information on this protein is inferred from genetics and molecular biology studies, whereas few data are available on the native protein. We previously reported (Salvatori et al., Biochem. Biophys. Res. Commun., 203, 1365-70, 1994) that MtPK is associate to membranes derived from terminal cisternae of the sarcoplasmic reticulum, from where it can be removed by alkali-treatment, similarly to other proteins like for instance, dystrophin, which is easily removed from sarcolemma at alkaline pH. We also reported (Salvatori et nl., 1. Muscle Res. Cell Mot& 16, 190, 1994) that MtPK is localized to the I- band and that quantitative differences exist among fast and slow skeletal and cardiac muscles. We decided to reinvestigate the aspect of fibre typing to search for any difference in reactivity. With the help of antibodies directed against specific markers like myosin heavy chains, we found that, in skeletal muscle, MtPK was maximally expressed in type I fibres, as expected, but also that type IIB fibres were unreactive and type IIA fibres had an intermediate level of reactivity. This seems to suggest a possible link between fibre metabolism and MtPK expression, Moreover, to better localize the Mtl’K within muscle fibres we isolated single skinned fibres for the observation with a laser confocal microscope. Results show that both slow and fast fibres exhibited the same banding pattern transverse to the long axis of the fibre, that the reactivity was due to regularly spaced spots and that the signal is reinforced in the correspondence of the external membrane. In the cardiac muscle fibres the staining seems to be rather different in that, the immunoreactivity appears to be confined to the intercalated disks.