Infection with the Endonuclear Symbiotic Bacterium Holospora obtusa Reversibly Alters Surface Antigen Expression of the Host Paramecium caudatum

It is known that the ciliate <i>Paramecium</i> cell surface including cilia is completely covered by high-molecular-mass GPI-anchored proteins named surface antigens (SAgs). However, their functions are not well understood. It was found that ciliate <i>Paramecium caudatum</i> reversibly changes its SAgs depending on the absence or presence of the endonuclear symbiotic bacterium <i>Holospora obtusa</i> in the macronucleus. Immunofluorescence microscopy with a monoclonal antibody produced SAg of the <i>H. obtusa</i>-free <i>P. caudatum</i> strain RB-1-labeled cell surface of the <i>H. obtusa</i>-free <i>P. caudatum</i> RB-1 cell but not the <i>H. obtusa</i>-bearing RB-1 cell. When this antibody was added to the living <i>P. caudatum</i> RB-1 cells, only <i>H. obtusa</i>-free cells were immobilized. An immunoblot with SAgs extracted from <i>Paramecium</i> via cold salt/ethanol treatment showed approximately 266-kDa SAgs in the extract from <i>H. obtusa</i>-free cells and 188 and 149-kDa SAgs in the extract from <i>H. obtusa</i>-bearing cells. <i>H. obtusa</i>-free RB-1 cells produced from <i>H. obtusa</i>-bearing cells via treatment with penicillin-G-potassium re-expressed 266-kDa SAg, while the 188 and 149-kDa SAgs disappeared. This phenotypic change in the SAgs was not induced by degrees of starvation or temperature shifts. These results definitively show that <i>Paramecium</i> SAgs have functions related to bacterial infection.