Turnover of proteins in myelin and myelin-like material of mouse brain

Abstract

Mice 14 or 60 days of age were injected intraperitoneally with [ 3H]leucine and were decapitated 10, 20, 40, and 80 days later. A crude fraction of brain myelin was isolated on a discontinuous sucrose gradient. Myelin was separated from the associated myelin-like material by osmotic shock and the two fractions were purified by differential centrifugation and isolated on a continuous CsCl gradient. Proteins of each subcellular fraction were separated by discontinuous gel electrophoresis in buffers containing sodium dodecyl sulfate and the specific activity of individual proteins determined. Myelin contained (in order of increasing molecular weight) two basic proteins, a protein doublet, proteolipid protein and a group of high molecular weight proteins. When animals were labeled at 14 days of age the radioactivity incorporated into the basic and proteolipid proteins of myelin was very stable metabolically (half-life much greater than 100 days), while the high molecular weight myelin proteins turned over with a half-life of about 70 days. The proteins of myelin labeled at 60 days of age turned over more rapidly, the half-life of basic and proteolipid proteins being about 95 days and that of the high molecular weight proteins about 40 days. The myelin-like material consisted primarily of high molecular weight proteins, although small amounts of basic and proteolipid protein also appeared to be intrinsic components. All the proteins of myelin-like materials turned over with a half-life of about 20 days, regardless of whether label was incorporated at 14 or 60 days of age. Thus, whereas the myelin basic and proteolipid proteins of both young and mature animals turn over very slowly as compared to the high molecular weight protein, all proteins in the myelin-like material turn over at the same, much faster rate in both young and mature animals.