Turnover of cytoskeletal proteins in vivo.

The turnover of the microtubule-associated proteins 1B and 2 (MAP1B and MAP2), tubulin, high molecular weight neurofilament protein (NF-H), and spectrin were studied by in vivo labeling. Radiolabeled [35S]methionine was injected intracranially to 10-day-old rats and the rate of turnover was measured for total and specific brain proteins. The turnover of total brain proteins was biphasic and consisted of a fast and a slow component with half lives of 6.5 +/- 0.4 and 14.2 +/- 0.7 (mean +/- S.E.M.) days, respectively. The turnover of individual cytoskeletal brain proteins was also biphasic. The fast decay rates of MAP1B, MAP2, tubulin and spectrin were 5.8 +/- 0.7, 6.9 +/- 0.3, 4.8 +/- 0.5 and 4.9 +/- 0.4 days, respectively, while the slow decay rates of these proteins were 12.0 +/- 1.3, 12.4 +/- 1.7, 15.0 +/- 0.5 and 16.0 +/- 1.2 days, respectively. In addition, the Triton X-100 insoluble fraction of MAP1B, tubulin, spectrin and NF-H showed monophasic decay rates of 29.0 +/- 2.3, 15.0 +/- 1.4, 16.0 +/- 0.9 and 18.5 +/- 1.5 days, respectively, which were similar to their slow decay rates in whole brain homogenates, suggesting that incorporation of these proteins into the cytoskeletal lattice increases their stability.