AGGREGATION OF PHOSPHOLIPID-VESICLES INDUCED BY THE RIBOSOME-INACTIVATING PROTEIN SAPORIN

Saporin-SG(SO-6) is a single chain ribosome inactivating protein, whic h can inhibit protein synthesis by inactivating eukaryotic ribosomes. The interaction of SO-6 with phospholipid model systems was described. SO-6 can specifically interact with negatively-charged phospholipid v esicles and it induces the aggregation of rhs lipid vesicles. The kine tics of the vesicle aggregation induced by SO-6 was studied. The satur ating protein/lipid molar ratio was determined to be 1:100 based on ti tration experiments. The aggregation is dependent on the temperature i n a range that was many times higher than the phase transition tempera ture of the phospholipid. The effect of pH on the aggregation of the v esicles can not be explained by simple deprotonation of side chain ami no groups of the protein, and may be related to conformational changes of the protein. The maintenance of physiological ionic strength was r equired for the aggregation of SO-6 with vesicles. Finally, the intera ction was prompted by Ca2+ ions, and was totally inhibited by EDTA, wh ich suggests that SO-6 may interact with phospholipid vesicles in a Ca 2+-dependent manner.