Studies on Subcellular Fractions Which Are Involved in Myelin Assembly: Labeling of Myelin Proteins by a Double Radioisotope Approach Indicates Developmental Relationships

Abstract

The question of developmental relationships among myelin-related membranes in subfractions of myelinating mouse brain (15 days) was investigated by a time-staggered double isotope protocol using [3H]leucine and [14C]leucine. Preliminary results are interpreted and discussed in the context of a mathematical conceptualization of pulse-labeling kinetic analyses of myelin proteins in subcellular membrane compartments. Differences in ratio of the 2 leucine labels among proteins of myelin-containing subfractions are interpreted as confirming metabolic differences relating to various stages of development rather than precursor-product relationships. The incorporation into myelin of 14 K [Kdalton] 17 K and 18.5 K basic proteins (MBP) occurs with relatively short delay times, following their synthesis (< 5 min), and seems to occur simultaneously into all compartments. The 21.5 K MBP and the proteolipid protein require 10-14 min and 14-20 min, respectively. A scheme is presented to illustrate the probable assignment of subfractions to various myelin compartments during myelination, and to serve as a working hypothesis for studies on precursor-product relationships.