Decrease in levels and rates of synthesis of tubulin and actin in developing rat brain.

The cytoplasmic and particulate tubulin content of postnatal rat brains was determined at various stages of development. The amount of tubulin in the soluble fraction was found to increase after birth and levels off at the age of 10-15 days, while the total protein content is still increasing. Indeed, the percentage of tubulin in the soluble fraction is about 33% at birth, stays at this value until day 10, and then decreases to 20% between days 10 and 15. On the other hand, the rate of increase in the level of the particulate tubulin parallels that of the total particulate proteins, and hence there is no change in the percentage of particulate tubulin during brain development. There was close agreement between the tubulin values obtained by the [3H]-colchicine binding assay and those obtained by electrophoretic resolution in sodium dodecylsulfate-polyacrylamide gels. Polyacrylamide gel electrophoresis was also utilized to determine actin levels in developing brains. The percentage of cytoplasmic brain actin also decreased with the age of the rats, from a value of 20% at birth to 10% at day 30, while the percentage of the particulate actin remained constant. The decline in the percentage of cytoplasmic tubulin and actin during brain development can be accounted for by reduction in the proportions of the respective mRNA species. Translation of poly (A)-rich brain mRNA in a wheat-germ cell-free system showed that the percentages of tubulin and actin synthesized decreased gradually with age. Similar results were obtained by analyzing the proteins produced by isolated brain polysomes in a brain cell-free system.