The gene for an abundant parasite coat protein predicts tandemly repetitive metal binding domains

Immobilization antigens are highly abundant surface membrane proteins that coat the surface of hymenostomatid ciliates. While their function is unknow n, recent studies with the common fish parasite, Ichthyophthirius multifili is, suggest their involvement in a novel mechanism of humoral immunity invo lving an effect of antibody on parasite behavior. To gain further insight i nto the nature of these proteins, we have cloned a gene encoding the 48 kDa i-antigen of I. multifiliis. Analysis of the gene (designated IAG48[G1]) r eveals a single, uninterrupted reading frame that predicts a protein of 442 amino acids. Based on its deduced amino acid sequence, the protein contain s hydrophobic amino acid domains at its N- and C-terminus that are characte ristic of signal peptide and GPI-anchor addition sites, respectively. The m ost striking feature of the predicted protein, however, is a series of tand em repeats that spans most of its length. The repeats themselves are charac terized by periodic cysteine residues that fall into register when the homo logous segments are aligned. interestingly, the spacing of cysteines (C-X-2 ,X-3-C) within a framework of larger (C-X-2-C-X-20-C-X3C-X-20-C-X2C) motifs is entirely consistent with the structure of known zinc-binding proteins. Finally, comparison of the coding sequence of the 48 kDa i-antigen gene wit h a partial cDNA previously thought to encode this protein reveals nearly c omplete identity except at their 3' ends, suggesting that alternative forms of the antigen exist. (C) 1999 Elsevier Science B.V. All rights reserved.