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MIC2 is an adhesive protein that participates in host cell invasion by the obligate intracellular parasite Toxoplasma gondii. Earlier studies established that MIC2 is secreted into the culture medium by extracellular parasites and that release is coincident with proteolytic modification. Since little is known about proteolytic processing of proteins secreted by T. gondii, we undertook this study to investigate the proteolytic events that accompany secretion of MIC2. We demonstrate that the C-terminal domain of MIC2 is removed by a protease, termed MPP1, when MIC2 is released into the culture supernatant. Additionally, prior to release, a second protease, termed MPP2, trims the N terminus of MIC2, resulting in the release of heterogeneously sized species of MIC2. Although MPP1 activity was unaffected by any of the protease inhibitors tested, MPP2 activity was blocked by a subset of serine and cysteine protease inhibitors. These results establish that MIC2 is proteolytically modified at multiple sites by two distinct enzymes that probably operate on the parasite surface.
Carruthers et al. (Mon,) studied this question.