Phosphoinositide binding profiles of the PX domains of Giardia lamblia

The membrane trafficking machinery that functions at the endomembrane system of Giardia lamblia appears to be significantly different from that present in most model eukaryotes. This machinery is important for encystation as cyst wall material is trafficked to the cell surface via encystation-specific vesicles. Since proteins containing the phosphoinositide-binding PX domains are known regulators of vesicular trafficking, BLAST search was used to identify the PX domains of G. lamblia. Six putative PX domain-containing ORFs were identified. Some of the encoded PX domains contained non-canonical amino acid residues in the highly conserved ligand binding pocket. In vitro and in vivo binding studies indicate that these domains have the ability to bind to diverse phosphoinositides. Also, coincidence detection is likely to play a significant role in ligand binding in vivo since domains that bind to the same lipid in vitro, exhibit differences in subcellular localization. Analyses of the expression of these six genes in trophozoites, encysting trophozoites and cysts showed that while the expression of four of the genes were downregulated in cysts, the other two were upregulated. The variation in ligand preference of the individual PX domains and the differential expression of most of the PX-domain encoding genes indicate that these PX domain-containing proteins are likely to perform diverse cellular functions.