Analysis by systematic deletion of amino acids of the action of the ribotoxin restrictocin.

A series of contiguous deletions were made in a cDNA encoding the ribonuclease restrictocin with the purpose of identifying the amino acids that are essential for the cleavage of the phosphodiester bond on the 3' side of G4325 in the alpha-sarcin/ricin domain of mammalian (rat) 28S rRNA. In all 93 of 149 amino acids, 62% of the residues in restrictocin, were not essential for the action of the toxin. Of the five residues that have been proposed to constitute the active site, three could be deleted without loss of activity if they were part of a deletion of three or five amino acids but not if they were removed singly. It is likely that the loss of these three residues is compensated for by a neighboring residue that occupies the structural space created by the larger amino acid deletions. This was demonstrated to be the case for the active site residue Glu95 which in the deletion mutant Delta91-95 is replaced by Asp90. Systematic deletion of amino acids is a rapid, cost effective method for identifying the residues in a protein likely to contribute directly to function and, hence, deserving of closer scrutiny. Moreover, a semiquantitative estimate of the contribution of the residue to function can be made. For this reason the method may be useful for functional proteomics.