Arginine 121 is a crucial residue for the specific cytotoxic activity of the ribotoxin alpha-sarcin

alpha -Sarcin, a cyclizing ribonuclease secreted by the mould Aspergillus g iganteus, is one of the best characterized members of a family of fungal ri botoxins. This protein induces apoptosis in tumour cells due to its highly specific activity on ribosomes. Fungal ribotoxins display a three-dimension al protein fold similar to those of a larger group of microbial noncytotoxi c RNases, represented by RNases T1 and U2. This similarity involves the thr ee catalytic residues and also the Arg121 residue, whose counterpart in RNa se Tl, Arg77, is located in the vicinity of the substrate phosphate moiety although its potential functional role is not known. In this work, Arg121 o f alpha -sarcin has been replaced by Gln or Lys. These two mutations do not modify the conformation of the protein but abolish the ribosome-inactivati ng activity of alpha -sarcin. In addition, the loss of the positive charge at that position produces dramatic changes on the interaction of alpha -sar cin with phospholipid membranes. It is concluded that Arg121 is a crucial r esidue for the characteristic cytotoxicity of alpha -sarcin and presumably of the other fungal ribotoxins.