"This particular protein, TgGAP50, was probably the major discovery here, an integral membrane protein, a protein embedded in the membrane of the parasite."
The researchers found that TgGAP50 associates with another major protein expressed by the parasite TgMyoA. Myosins are known to be involved in motility. For example, they are present in muscle, where, in combination with the protein actin, they form the thick filaments of muscle.
"This new protein is embedded in the inner membrane complex of the parasite, where it's directly involved in anchoring myosin to the membrane," Beckers said. "This is, in fact, only the second example of a protein that directly does this."
Thus, the new protein is a specific membrane receptor for what the researchers say is a "myosin motor."
Toxoplasma motility may be a result of the myosin moving along the length of actin filaments in the parasite, Beckers said. Alternatively, it may be caused by the myosin holding onto the end of a growing actin filament. Either way, the myosin molecule needs to be anchored in the parasite for movement to occur.
"If the myosin is not anchored anywhere, its movement with respect to an actin filament will not result in parasite motility," Beckers said. "As an analogy, if you're sitting in a small boat and throw a rope out to the dock and someone's there to hold it, you can pull yourself toward that person. But if no one is there, all you'll do is pull the rope and no net movement will occur."
Thus, apart from having identified a complex of proteins containing a major myosin in Toxoplasma, the new study has "gone one step further because we identified a protein that actually anchors this myosin-containing complex in the membrane. And this protein is absolutely critical to parasite motility," Beckers said.
"Since motility is so central to survival of this class of parasites, it'
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Contact: Leslie H. Lang
llang@med.unc.edu
919-843-9687
University of North Carolina School of Medicine
21-May-2004