INVESTIGATION OF MYOTONIC-DYSTROPHY KINASE ISOFORM TRANSLOCATION AND MEMBRANE ASSOCIATION

Myotonic dystrophy is caused by the expansion of a CTG repeat found in the 3'-untranslated region of the myotonic dystrophy kinase. The mech anism of disease and the role of the kinase are currently obscure. Her e we begin the investigation of domain structure/function correlations to aid in determining its normal function. Expressed full-length prot ein and protein truncated before a C terminal hydrophobic domain were compared. In vitro, signal peptide function and protection of kinase b y microsomal membranes were absent; thus, it is not translocated, as p reviously proposed. However, full-length kinase expressed in insect ce lls was found in fractions enriched for membranes and decorated mitoch ondria. The truncated form was found primarily in the cytosol. The kin ase was present as two self-associated, disulfide-linked complexes, Th e majority of full-length kinase was found in the larger of the two co mplexes, while almost all of the truncated form was found in the small er. Thus, the C-terminal region confers a higher order of self-associa tion. Furthermore, full-length kinase expressed in COS-1 cells was pre sent as high molecular weight complex, while the truncated form was pr esent as monomer species. These experiments indicate that the myotonic dystrophy kinase is not membrane-integrated but that it may have a mo lecular organization which favors peripheral association with membrane s.