Systematic deletion analysis of ricin A-chain function. Single amino acid deletions.

The A-chain of ricin is a cytotoxic RNA N-glycosidase that inactivates ribosomes by depurination of the adenosine at position 4324 in 28 S rRNA. Of the 267 amino acids in the protein, 222 could be deleted, in one or another of 74 mutants, without the loss of the capacity to catalyze hydrolysis of a single specific nucleotide in rRNA (Morris, K. N., and Wool, I. G. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 4869-4873). The 45 amino acids that could not be omitted when the deletions were in sets of 20, 5, or 2 residues have now been deleted one at a time; 9 of these deletion mutants retained activity. A RNP-like structural motif in ricin A-chain that may mediate binding to ribosomal RNA has been identified.