Involvement of the amino acids outside the active-site cleft in the catalysis of ricin A chain

The A chain of ricin (RA) is a cytotoxic RNA N-glycosidase that inactivates ribosomes by depurination of the adenosine residue at position 4324 in 28S rRNA. Of the 267 amino acids in the protein, 231 could be deleted in one or another of 83 mutants, without the loss of the capacity to catalyze hydrolysis of a single specific nucleotide in rRNA [Morris, K. N. sr Wool, I. G. (1992) Proc. Natl Acad. Sci. USA 89, 4869-4873]. Expression of 29 selected deletion mutants of RA in prokaryotic cell-free coupled transcription-translation reactions was carried out and the activities of the mutants were assessed by monitoring depurination of reticulocyte ribosomes. Kinetic analysis of five deletion mutants which retained detectable activity was performed. Deletion of amino acids outside the putative active-site cleft in these mutants significantly affected the catalytic rate rather than the interaction with ribosomes. From these data, the amino acids far from the active-site cleft appeared to be involved in alignment of the key residues for catalysis and interaction with the target tetraloop structure of 28S rRNA.