The in vivo synthesis of myelin proteins in rabbit sciatic nerve

Rabbits were injected into the sciatic nerves with either (35)S-methionine, or (3)H-fucose. After times ranging from 45 min to 15 days the nerves were removed and the total particulate material from the nerves fractionated to give seven subfractions with densities between 0.2 and 1.2 M sucrose. The patterns of radio-labelled proteins were examined by SDS-PAGE and quantitative fluorography. The results showed that the P(2) basic protein was metabolically far more active than either the major P(0) glycoprotein, or the basic protein BP. The P(2) protein also entered the myelin fractions more rapidly than either P(0), or BP components. The net synthesis of P(0) was slower than P(2) and BP and this intrinsic membrane protein remained associated with the denser membrane fractions (>0.7 M sucrose) for longer than the basic proteins prior to entering myelin. Newly synthesized high molecular weight proteins remained concentrated in the denser membrane fractions and turned over faster than the myelin proteins. A low density myelin fraction (B) was detected in which both the P(2) protein and certain high molecular weight proteins became more rapidly labelled than in compact myelin. In this fraction the specific activity remained higher than that of compact myelin for up to five days after the injection of (35)S-methionine into the nerve. The results indicate that the major PNS myelin proteins are incorporated into and turn over in the various compartments of the Schwann cell plasma membrane-myelin continuum at very different rates.