Arginine Deiminase May Regulate Surface Protein Citrullination and Differentiation in the Primitive Eukaryote Giardia lamblia.

MARIA CAROLINA TOUZ

Mendoza

Congreso; VII Congreso Argentino de Protozoología y Enfermedades Parasitarias.; 2005

SAP

Giardia lamblia undergoes antigenic variation, a process where a single variant-specific surface protein (VSP) is replaced by an antigenically-different VSP. All VSP genes (~150) encode type I integral membrane proteins that have a conserved five-amino acid cytoplasmic tail, CRGKA. To identify molecules interacting with this tail, the peptide His6-CRGKA was employed in pull-down assays. Surprisingly, arginine deiminase (gADI) bound specifically to CRGKA as identified by mass spectroscopy.  In Giardia and prokaryotes, arginine deiminase is known to catalyze the conversion of free arginine into citrulline. In contrast, higher eukaryotes possess peptidil-arginine deiminases (PAD), enzymes that post-translationally deiminates the guanidine group of arginine residues in peptides and proteins to generate ammonia and citrulline. Analysis of Giardia proteins using a citrulline-specific antibody showed that VSPs are citrullinated. Activity analysis using His6-CRGKA as substrate and purified gADI, demonstrated that this peptide can be citrullinated in vitro (CcitGKA). Immunofluorescence analysis of HA-tagged gADI showed cytoplasmic localization and close association to the plasma membrane. When trophozoites were induced to differentiate into cysts, gADI is translocated from the cytoplasm to the nuclei and the process of encystation was abolished. These results show that citrullination plays an essential role in both defense mechanisms of Giardia, encystation and antigenic variation.