Evidence for defective incorporation of proteins in myelin of the quaking mutant mouse

Abstract

The defect in myelinogenesis present in the Quaking mutant mouse was investigated using a double radioisotope technique for comparing the incorporation of amino acid into myelin proteins of normal and mutant mice. Quaking mice and littermate controls recieved intracranial injections of 150 μCi [ 3H]glycine and 25 μCi of [ 14C]glycine respectively. After 2 h their brains were combined and jointly processed to obtain subcellular fractions. The 3H/ 14C ratio for the myelin subfraction was 1.88 as compared to a 3H/ 14C ratio of 3.0 for the other subfractions, indicating a 40% decrease in glycine incorporation into myelin of Quaking mice. Myelin proteins were separated by discontinuous gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) and the 3H/ 14C ratios determined in each gel slice. In contrast to the microsomal subfractions which gave a 3H/ 14C ratio of 2.6 across the gel, the 3H/ 14C ratio of myelin showed large variations with values ranging from 0.54 for proteolipid protein to 2.0 for some of the high molecular weight proteins. During development, the Quaking mutant exhibited a preferential depression in glycine incorporation into proteolipid protein in 18-day-old mice, while in older animals (32–54 days) the fast migrating basic protein, as well as the proteolipid protein, was labeled to a significantly lesser extent.