SORTING WITHOUT A GOLGI COMPLEX: THE ENDOPLASMIC RETICULUM AS THE SORTING CORE FACILITY IN THE PROTOZOAN GIARDIA LAMBLIA

ZAMPONI, NAHUEL; TOUZ, MARÍA CAROLINA

Salta

Congreso; LV SAIB - XIV PABMB.; 2019

SAIB

Our understanding of protein and lipid trafficking in eukaryotic cells has been challenged by the finding of different forms of compartmentalization and cargo processing in protozoan parasites. Giardia lamblia is a parasitic organism that belongs to the Excavata, an early diverging group within Eukarya. Both early divergence (contingency) and parasitism (selective pressure) are evolutionary forces that have given rise to the unique feature of this organism. Giardia lacks many organelles that are considered hallmarks of the eukaryotic organization, such as mitochondria, peroxisomes, endosomes, and the Golgi apparatus. Moreover, the Giardia genome is small, has very few introns, and lacks many genes that are considered essential for the maintenance of the eukaryotic cellular machinery. Our studies showed that, in the absence of a Golgi compartment in Giardia, proteins destined for secretion are directly sorted and packaged at specialized ER regions enriched in COPII coatomer complexes and ceramide. We also demonstrated that ER-resident proteins are retained at the ER by the action of a KDEL receptor, which, in contrast to other eukaryotic KDEL receptors, showed no interorganellar dynamic but instead acts specifically at the limit of the ER membrane. Our results suggest that the ER-exit sites and the perinuclear ER-membranes are capable of performing protein sorting functions. These findings support the vision that Giardia adaptation represents an extreme example of reductive evolution without loss of function.