Synthesis of Lens Protein in vitro

N‐terminal acetylation of the lens protein α‐crystallin has been studied in a lens cell‐free system. This system is capable of synthesizing de novo α‐crystallin, a protein with acetylmethionine as N‐terminus. When [ 35 S]Met‐tRNA fMet is used as radioactive precursor only the α‐crystallin chains are labeled in the N‐terminal position. This phenomenon enables the study of the process of acetylation. Newly synthesized polypeptides could be separated according to chain length. Electrophoretic analysis of thermolytic peptides revealed that acetylation of the N‐terminus occurs while the chain is still on the ribosome. It is concluded that the N‐terminal acetylation takes place after chain initiation and before completion of the polypeptide chain.