Studies on the initiation of protein synthesis in mouse myeloma tumors.

“Sand 3H-labeled short, nascent peptides have been extracted from mouse myeloma ribosomes after incubation of myeloma fragments with [?S]methionine and r3H]amino acids. Edman analysis of these peptides reveals that most of the methionine is present at the N terminus but that other N-terminal amino acids are also present. Light chains synthesized by the RPC-20 tumor fragments were purified from ribosomes, cell sap (released light chain) and the incubation medium (secreted light chain). Ribosomebound light chains were found to possess some N-terminal methionine whereas released and secreted light chains did not. Since methionine is not the N-terminal amino acid of light (L) chains purified from the urine of tumor-bearing mice, the results indicate that methionine initiates L chain biosynthesis in the myeloma. Total nascent %-labeled peptides were extracted from myeloma ribosomes and fractionated on Sephadex G-50. Edman analysis of chromatographic fractions of varying sizes indicated that the percentage of N-terminal methionine decreased with increasing chain length. This behavior is expected if methionine serves as an initiator amino acid but is removed before completion of the polypeptide chain on the ribosome.