Molecular characteristics of an immobilization antigen gene of the fish-parasitic protozoanIchthyophthirius multifiliisstrain ARS-6

Abstract

AbstractIchthyophthirius multi¢liis(Ich), a ciliated protozoanparasite of ¢sh, expresses surface antigens (i-anti-gens), which react with host antibodies that renderthemimmobile.Thenucleotidesequenceofani-anti-gen gene of I. multi¢liis strain ARS-6 was deduced.The predicted protein of 47493Da is comprised of460 amino acids (aa’s) arranged into ¢ve imperfectrepeats with periodic cysteine residues with thestructure: CX (19)20 CX 2 CX 16 27 CX 2 CX 20(21) CX 3 . TheN-terminal aa’s typify a signal peptide motif whilea stretch of C-terminal aa’s resemble a glycosyl^phosphatidyl^inositol (GPI)-anchor addition site.Thedegreeofdeducedi-antigenaasequenceidentityof strain ARS-6 (GenBank accession # ACH87654and # ACH95659) with other I. multi¢liis i-antigensequences present in GenBank ranges from 99% to36% identity with 52kDa i-antigens of I. multi¢liisstrain G5 (accession #s AAK94941and AAK01661respectively). Immunoblot analysis of i-antigens fol-lowing exposure of I. multi¢liis theronts to cat¢shanti-I. multi¢liis immune serum did not show anyappreciable alteration in i-antigen expression. Themechanism that regulates i-antigen expression inI.multi¢liisremainsapuzzlingquestion.Keywords: immobilization antigen, gene, nucleo-tidesequence,Ichthyophthiriusmulti¢liisIntroductionIchthyophthiriusmulti¢liis(Ich) is a hymenostomatidparasitic ciliate that causes ‘white spot’ disease inmany species of freshwater cultured and/or free liv-ing ¢sh. Fish that recover from I. multi¢liis infectionareknowntopossessbothserumandcutaneousmu-cus antibodies that are capable of immobilizing theparasite in vitro (Clark & Dickerson 1997). The anti-gens on the parasite surface that are involved in theantibody-mediatedimmobilizationarecomprisedofaclass of abundant glycosyl^phosphatidyl^inositol(GPI)-anchoredmembraneproteinsthatareaptlyde-signated as immobilization antigens (i-antigens)(Clark, Gao, Gaertig, Wang & Cheng 2001; Clark FHatanka,Umeda,YamashitaH Swennes, Findly DLee & Kim 2008).Tetrahymena and Paramecium spe-cies are free-living ciliated protists that have servedas model systems, in which i-antigens have been ex-tensively studied (Doerder 2000; Simon & Schmidt2005). Both Tetrahymena and Paramecium possess arepertoire of paralogous genes that encode alterna-tive forms of i-antigens, the expression of which isunder the in£uence of certain environmental cuessuch as temperature and medium, in which they