SWITCHING OF PARAMECIUM SURFACE-ANTIGEN TYPES WITH PURIFIED ANTIGEN AND CONDITIONED MEDIUM CONTAINING 70 KD PROTEINS

Paramecia can be induced to change the kind of i protein characteristi c of their surfaces by either cell contact with paramecia of a differe nt serotype or after exposure to cell-free conditioned media in which growth has occurred. We have examined these media by SDS-polyacrylamid e gel electrophoresis and have detected surprisingly few proteins. One protein which is present only in media which can switch serotypes has an apparent molecular weight of about 70 kD. The amount of this prote in as judged by Coomassie blue staining is correlated with the effecti veness of a medium in eliciting antigenic switching. The 70 kD protein s are found not only in media which transform but also in ''purified'' surface antigen preparations, as shown by immunoblots using antisera prepared against 70 kD proteins isolated from antigen by gel filtratio n. Antibodies to these proteins generally are detectable by enzyme-lin ked immunosorbent assay in all antisera made against surface antigen.