L. Lecordier et al., Transmembrane insertion of the Toxoplasma gondii GRA5 protein occurs aftersoluble secretion into the host cell, MOL BIOL CE, 10(4), 1999, pp. 1277-1287
The intracellular parasite Toxoplasma gondii resides within a specialized c
ompartment, the parasitophorous vacuole (PV), that resists fusion with host
cell endocytic and lysosomal compartments. The PV is extensively modified
by secretion of parasite proteins, including the dense granule protein GRA5
that is specifically targeted to the delimiting membrane of the PV (PVM).
We show here that GRA5 is present both in a soluble form and in hydrophobic
aggregates. GRA5 is secreted as a soluble form into the PV after which it
becomes stably associated with the PVM. Topological studies demonstrated th
at GRA5 was inserted into the PVM as a transmembrane protein with its N-ter
minal domain extending into the cytoplasm and its C terminus in the vacuole
lumen. Deletion of 8 of the 18 hydrophobic amino acids of the single predi
cted transmembrane domain resulted in the failure of GRA5 to associate with
the PVM; yet it remained correctly packaged in the dense granules and was
secreted as a soluble protein into the PV. Collectively, these studies demo
nstrate that the secretory pathway in Toxoplasma is unusual in two regards;
it allows soluble export of proteins containing typical transmembrane doma
ins and provides a mechanism for their insertion into a host cell membrane
after secretion from the parasite.