MONOCLONAL-ANTIBODY TO A BACTERIAL ENDONUCLEAR SYMBIONT HOLOSPORA CROSS-REACTS WITH PROTEINS OF CONTRACTILE VACUOLE RADIAL CANALS OF PARAMECIUM SPECIES

A monoclonal antibody (mAb) IR-2-1 was raised against a 67-kDa protein purified from the macronucleus-specific bacterial symbiont Holospora obtusa of Paramecium caudatum. The mAb was found to react with two ban ds (31 and 67-kDa) on gels of H. obtusa. Indirect immunofluorescence m icroscopy showed that these antigens were distributed inside the cells . However, unexpectedly, this mAb also cross reacted with the radial a rms of the contractile vacuole in P. caudatum, P. tetraurelia, P. mult imicronucleatum, P. jenningsi and P. bursaria as well as with their cy toplasm. Immunoelectron microscopy showed that the antigens were locat ed on the decorated spongiome of the radial arms. In immunoblots, mAb IR-2-1 reacted with a band of 67 kDa in all Paramecium species examine d. However, no band appeared in the immunoblot of isolated macronuclei of H. obtusa-free P. caudatum and no label was seen in the nuclear ma trix of the macronucleus of air-dried P. caudatum. These results sugge st that the 67-kDa antigen found in H. obtusa was not imported from th e host macronucleus and the same antigen in the host contractile vacuo les and cytoplasm were not derived from the symbiont. These results al so showed that an epitope on the decorated spongiome of the Paramecium species is shared by its bacterial symbiont. In contrast to the decor ated tubule-specific mAb, DS-1, the antigens for IR-2-1 appeared to be loosely membrane bound as they were lost in paraformaldehyde fixed an d acetone permeabilized Paramecium.