The above experiments strongly suggest that microneme proteins are involved in cell recognition and/or attachment and entry. To examine directly the binding of microneme proteins to host cells, we used MIC2 as a surrogate marker and tested the ability of both secreted microneme proteins contained in excretory±secretory antigen (ESA) preparations and microneme proteins isolated from semi-puri®ed micronemes to bind to HFF cells. Binding was analysed by incubation of host cell monolayers with protein samples at 128C to avoid possible uptake by endocytosis. After incubation with ESA or puri®ed microneme proteins, the supernatant was removed, cells were washed extensively, and thecell-boundfraction(CBF)was isolated by detergent lysis. The presence of MIC2 in the unbound fraction, the washes and the CBF was compared by SDS±PAGE and Western blotting. MIC2 was abundant in both the ESA and the microneme protein preparation that were recovered in the supernatants after incubation (referred to as SUP in Fig. 7). MIC2 was also detected in the CBF from the microneme protein preparation, but not in the corresponding CBF from the ESA fraction (Fig. 7). Phosphorimage analysis in comparison with dilution standards that were loaded on the same gel revealed that <2±3% or 20±30ng of the input MIC2 protein was bound under these conditions. As a negative control, we also analysed the binding of GRA1, a protein that is found in ESA and is a contaminant of the microneme fraction, but is not thought to bind to host cells. Although it was readily detected in the supernatant fraction, little or no GRA1 was found in either CBF.
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