The microneme protein MIC4, or an MIC4-like protein, is expressed within the macrogamete and associated with oocyst wall formation in Toxoplasma gondii.

The expression and localisation of MIC4, or an immuno-cross reacting MIC4-like protein, was examined in the enteric forms of Toxoplasma gondii using immunocytochemistry. In addition to being located within the micronemes of the merozoites, MIC4 or the MIC4-like protein was present within the macrogamete and was associated with the developing oocyst wall. The macrogamete is characterised by two types of structurally distinct wall forming bodies (WFB1 and 2). However, by immuno-electron microscopy, it was possible to identify two populations of dense granules (WFB1) which appear to form sequentially during macrogamete development. The first granules to form (WFB1a) stained positively with anti-MIC4 and were followed by MIC4 negative granules (WFB1b). During oocyst wall formation, the WFB1a and b sequentially released their contents onto the surface with WFB1a material forming an anti-MIC4 positive outer veil, while the WFB1b forms the electron dense outer layer of the oocyst wall. The inner layer was formed by WFB2. Thus, for the first time, it was possible to identify two populations of dense granules (WFB1a and b) involved in the formation of different parts of the oocyst wall. It was not possible to analyse the contents of macrogametes by western blot to unequivocally identify the antigen recognised by the polyclonal antisera as MIC4.