Hydrophobic and hydrophilic radio-iodination, crosslinking, and differential extraction of cell surface proteins in Paramecium tetraurelia cells

We combined widely different biochemical methods to analyze proteins of the cell surface of P. tetraurelia since so far one can isolate only a subfrac tion of cell membrane vesicles enriched in the GPI-anchored surface antigen s ("immoblization" or "i-AGs"). We also found that i-AGs may undergo partia l degradation by endogenous proteases. Genuine intrinsic membrane proteins were recognized particularly with lipophilic 5-[I-125]-iodonaphthalene-1-az ide (INA) labeling which reportedly "sees" integral proteins and cytoplasmi c cell membrane-associated proteins. With INA (+DTT), bands of less than or equal to 55 kDa were similar as with hydrophilic iodogen (+DTT), but inste ad of large size bands including i-AGs, a group of 122, 104 and 94 kDa appe ared. Several bands of the non i-AG type are compatible with integral (poss ibly oligomeric) or associated proteins of the cell membrane of established molecular identity, as we discuss. In summary, we can discriminate between i-AGs and some functionally important minor cell membrane components. Our methodical approach might be relevant also for an analysis of some related protozoan parasites.