FACTORS FOR PROTEIN SYNTHESIS IN THE AXOPLASM OF SQUID GIANT AXONS

In the giant axon of the squid, axoplasmic RNA was identified as transfer RNA by its electrophoretic mobility in polyacrylamide gels and the capacity to be charged with radioactive amino acids. If this RNA is involved in axonal protein synthesis, the axoplasm should contain all the soluble factors required for the synthetic process. These factors were detected in the axoplasm of squid giant axons using 3H-leucine incorporation by rabbit reticulocytes, and confirmed by the presence of aminoacyl-tRNA synthetases and elongation factors in the axoplasm PMS (post-mitochondrial supernatant). Synthetase activity was assayed by the tRNA-dependent incorporation of a mixture of 14C-amino acids into an acid insoluble product. The assay used for the independent measurement of the soluble factors necessary for peptide chain elongation was based on the synthesis of polyphenyl-alanine in a poly(U)-programed cell-free system containing washed reticulocyte ribosomes, (14C) phenylalanyl-tRNA and increasing amounts of axoplasm PMS. Before axoplasmic soluble factors can be considered an integral part of an axonal system of protein synthesis, direct evidence is needed for the occurrence of axonal ribosomes. Without such evidence the soluble factors should be regarded as molecular components leaked from the neuronal perikarya into the axoplasmic stream or on their way to active sites of protein synthesis located in nerve terminals.