Synthesis of membrane protein in slices of rat cerebral cortex

Protein synthesis was studied in slices from rat cerebral cortex and localized in various purified subcellular membrane fractions isolated after incubation with L-leucine. A synaptosomal fraction isolated from a several times washed crude mitochondrial pellet showed very little contamination by free membranes, and the synaptic membrane fraction isolated from it was estimated to c +ain about 50% of this component. Leucine incorporation into all fractions was highly sensitive (greater than 95%) to emetine except for the cell and synaptic mitochondrial subfractions. They were only 60 to 70% inhibited by emetine and showed 10 to 20% inhibition by chloramphenicol which probably was due to those proteins synthesized in situ by the mitochondria. The net rate of incorporation of labeled protein into the synaptosomal and synaptic membrane subfractions was low, and axonal flow was excluded as a significant source of this label. On the basis of autoradiography it was concluded that the majority of the protein label was contributed by free and membrane-enclosed ribosome-containing contaminants in these fractions. Unlabeled and labeled subcellular fractions were also analyzed by electrophoresis in the presence of sodium dodecyl sulfate. The staining profiles of the microsomal and synaptic membrane subfractions were nearly identical, whereas those of synaptic mitochondria and the soluble proteins of the cell bodies were unique. The labeling of all proteins was blocked by emetine, except for the synaptic mitochondria (cell mitochondria were not examined). These products of mitochondrial protein synthesis exhibited three labeling peaks, the major one with a molecular weight of approximately 38,000. In the absence of emetine, slices incorporated amino acids into soluble and microsomal proteins with high specific activity. The labeled proteins of the soluble fraction were more highly concentrated in the range of molecular weights smaller than 50,000 than were those in the microsomal fraction, and it was concluded that a considerable portion of the protein-synthesizing machinery of the brain is geared to form soluble proteins. The gel patterns of labeled microsomal and synaptic membranes were consistent with contamination by the former contributing most of the protein label in the latter.