The I-antigens of Ichthyophthirius multifiliis are GPI-anchored proteins

The parasitic ciliate Ichthyophthirius multifiliis has abundant surface mem brane proteins (i-antigens) that when clustered, trigger rapid, premature e xit from the host. Similar antigens are present in free-living ciliates and are GPI-anchored in both Paramecium and Tetrahymena. Although transmembran e signalling through GPI-anchored proteins has been well-documented in meta zoan cells, comparable phenomena have yet to be described in protists. Sinc e premature exit of Ichthyophthirius is likely to involve a transmembrane s ignalling event, we sought to determine whether i-antigens are GPI-anchored in these cells as well. Based on their solubility properties in Triton X-1 14. the i-antigens of Ichthyophthirius are amphiphilic in nature and partit ion with the detergent phase. Nevertheless, following treatment of detergen t lysates with phospholipase CI the same proteins become hydrophilic. Conco mitantly, they are recognized by antibodies against a cross-reacting determ inant exposed on virtually all GPI-anchored proteins following cleavage wit h phospholipase C. Finally, when expressed in recombinant form in Tetrahyme na thermophila, full-length i-antigens are restricted to the membrane, whil e those lacking hydrophobic C-termini are secreted from the cell. Taken tog ether, these observations argue strongly that the i-antigens of Ichthyophth irius multifiliis are, in fact, GPI anchored proteins.