DIFFERENT CONFORMATIONS OF NASCENT PEPTIDES ON RIBOSOMES

The length at which the N terminus of nascent proteins becomes availab le to antibodies during their synthesis on ribosomes was determined. T hree different proteins, bovine rhodanese, bacterial chloramphenicol a cetyltransferase and MS2 coat protein, were synthesized with coumarin at their N terminus in a cell-free system derived from Escherichia col i. A derivative of coumarin was cotranslationally incorporated as N-co umarin-methionine at the N terminus of polypeptides. The interaction o f specific anti-coumarin antibodies with this N-terminal coumarin of r ibosome-bound nascent peptides was examined. The results indicate that short nascent peptides of each of the three proteins are unreactive, that the length at which they become accessible to the antibodies is d ifferent for the three proteins, and that longer peptides differ in th eir reactivity. It is suggested that these differences are due to diff erences in the conformation acquired by the peptides as they are synth esized on the ribosomes. (C) 1998 Academic Press Limited.