INCREASED EXPRESSION OF THE MOLECULAR CHAPERONE BIP GRP78 DURING THE DIFFERENTIATION OF A PRIMITIVE EUKARYOTE/

Giardia lamblia, a major cause of intestinal disease worldwide, is a p arasitic protozoan that represents the earliest branch of the eukaryot ic lineage. Trophozoites, which possess two nuclei but lack mitochondr ia, peroxisomes and a typical Golgi apparatus, colonize the small inte stine of the vertebrate host where they may differentiate into infecti ve cysts. Encystation is a regulated process characterized by the bios ynthesis, secretion and formation of a protective extracellular cyst w all. In previous studies, we demonstrated the biogenesis of the Golgi apparatus during encystation and identified two leucine-rich proteins (CWPs), which localize within encystation-specific secretory granules before their incorporation into the cyst wall. Here, we used immunolog ical, biochemical and molecular biological approaches to analyze the e xpression of BiP/GRP78, an endoplasmic reticulum (ER)-resident chapero ne, during the Giardia life cycle. A monoclonal antibody specific for Giardia BiP permitted the visualization of the ER of this protozoan an d showed that BiP expression increased simultaneously with the increas ed expression of CWPs during encystation. However, in contrast to the 140-fold increase in levels of CWP transcripts, the steady-state level of BiP mRNA did not increase during encystation. Furthermore, potent inducers of BiP expression in higher eukaryotic cells, including agent s that perturb the ER environment, did not affect BiP expression in Gi ardia. These results, when considered together with the profound chang es that occur in the secretory pathway during Giardia encystation, ind icate an important role for this molecular chaperone during the differ entiation of this primitive eukaryote.