Toxoplasma gondii micronemalprotein MIC1 is a lactose-binding lectin.

Host cell invasion by Toxoplasma gondii isa multistep process with one of the first steps being the apicalrelease of micronemal proteins that interact with host receptors.We demonstrate here that micronemal protein 1 (MIC1) is a lactose-bindinglectin. MIC1 and MIC4 were recovered in the lactose-eluted (Lac+)fraction on affinity chromatography on immobilized lactose of thesoluble antigen fraction from tachyzoites of the virulent RH strain.MIC1 and MIC4 were both identified by N-terminal microsequencing. MIC4was also identified by sequencing cDNA clones isolated from an expressionlibrary following screening with mouse polyclonal anti-60/70kDa (Lac+ proteins) serum. This antiserum localizedthe Lac+ proteins on the apical region of T. gondii tachyzoites by confocal microscopy. The Lac+ fractioninduced hemagglutination (mainly type A human erythrocytes), whichwas inhibited by β-galactosides (3 mMlactose and 12 mM galactose) but not by up to 100 mM melibiose (α-galactoside), fucose, mannose, orglucose or 0.2 mg/ml heparin. The lectin activity of theLac+ preparation was attributed to MIC1, becauseblotted MIC1, but not native MIC4, bound human erythrocyte typeA and fetuin. The copurification of MIC1 and MIC4 may have beendue to their association, as reported by others. These data suggestthat MIC1 may act through its lectin activity during T.gondii infection. [ABSTRACT FROM PUBLISHER]