The differential catalytic activity of ribosome-inactivating proteins saporin 5 and 6 is due to a single substitution at position 162

Saporin, a type I ribosome-inactivating protein produced by the soapwort plant Saponaria officinalis belongs to a multigene family that encodes its several isoforms. The saporin seed isoform 6 has significantly higher N-glycosidase and cytotoxic activities compared with the seed isoform 5, although the two have identical active sites. In the present study, we have investigated the contribution of non-conservative amino acid changes outside the active sites of these isoforms towards their differential catalytic activity. The saporin 6 residues Lys¹³⁴, Leu¹⁴⁷, Phe¹⁴⁹, Asn¹⁶², Thr¹⁸⁸ and Asp¹⁹⁶ were replaced by the corresponding saporin 5 residues, Gln¹³⁴, Ser¹⁴⁷, Ser¹⁴⁹, Asp¹⁶², Ile¹⁸⁸ and Asn¹⁹⁶, to generate six variants of saporin 6, K134Q, L147S, F149S, N162D, T188I and D196N. By functional characterization, we show that the change in amino acid Asn¹⁶² in saporin 6 to aspartic acid residue of saporin 5 contributes mainly to the lower catalytic activity of saporin 5 compared with saporin 6. The non-involvement of other non-conservative amino acids in the differential catalytic activity of these isoforms was confirmed with the help of the double mutations N162D/K134Q, N162D/L147S, N162D/F149S, N162D/T188I and N162D/D196N.