Post-translational modifications are able to regulate protein function and 
cellular processes in a rapid and reversible way. SUMOylation, the 
post-translational modification of proteins by the addition of SUMO, is a highly 
conserved process that seems to be present in modern cells. However, the 
mechanism of protein SUMOylation in earlier divergent eukaryotes, such as 
Giardia lamblia, is only starting to become apparent. In this work, we report 
the presence of a single SUMO gene encoding to SUMO protein in Giardia. 
Monoclonal antibodies against recombinant Giardia SUMO protein revealed the 
cytoplasmic localization of native SUMO in wild-type trophozoites. Moreover, the 
over-expression of SUMO protein showed a mainly cytoplasmic localization, though 
also neighboring the plasma membrane, flagella, and around and even inside the 
nuclei. Western blot assays revealed a number of SUMOylated proteins in a range 
between 20 and 120 kDa. The genes corresponding to putative enzymes involved in 
the SUMOylation pathway were also explored. Our results as a whole suggest that 
SUMOylation is a process conserved in the eukaryotic lineage, and that its study 
is significant for understanding the biology of this interesting parasite and 
the role of post-translational modification in its evolution.