DEVELOPMENTALLY-REGULATED EXPRESSION OF A GIARDIA-LAMBLIA CYST WALL PROTEIN GENE

The protozoan Giardia lamblia is an obligate parasite of the mammalian small intestine. We studied the expression of a gene that encodes a p rotein component of the cyst wall, a complex structure assembled durin g the differentiation of trophozoites to cysts and which is critical t o survival of the parasite outside its mammalian host. Transcripts fro m the cyst wall protein gene increase more than 100-fold during encyst ation, reaching a maximum between 5 and 24 hours after induction. Cyst wall protein expression also increases dramatically during encystatio n, and, prior to its incorporation into the nascent cyst wall, the pro tein is contained within the encystation-specific vesicles of encystin g trophozoites. The sequence of the cloned gene predicts an acidic, le ucine-rich polypeptide of M(r) 26000 that contains 5.3 tandemly arrang ed copies of a degenerate 24-amino-acid repeat. A hydrophobic amino-te rminal peptide probably serves as the initial signal that targets this protein to a secretory pathway involving vesicular localization durin g encystation and, ultimately, secretion to form the cyst wall.