The action mode of the ribosome-inactivating protein alpha-sarcin

Based on the tertiary structure of the ribosome-inactivating protein alpha- sarcin, domains that are responsible for hydrolyzing ribosomes and naked RN A have been dissected. In this study, we found that the head-to-tail intera ction between the first amino beta-strand and the last carboxyl beta-strand is not involved in catalyzing the hydrolysis of ribosomes or ribonucleic a cids. Instead, a four-strand pleated beta-sheet is indispensable for cataly zing both substrates, suggesting that alpha-sarcin and ribonuclease T1 (RNa se T1) share a similar catalytic center. The integrity of an amino beta-hai rpin and that of the loop L3 in alpha-sarcin are crucial for recognizing an d hydrolyzing ribosomes in vitro and in vivo, However, a mutant protein wit hout the beta-hairpin structure, or with a disrupted loop L3, is still capa ble of digesting ribonucleic acids. The functional involvement of the beta- hairpin and the loop L3 in the sarcin stem/loop RNA of ribosomes is demonst rated by a docking model, suggesting that the two structures are in essence naturally designed to distinguish ribosome-inactivating proteins from RNas e T1 to inactivate ribosomes, Copyright (C) 2000 National Science Council, ROC and S. Karger AG, Basel.