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A more recent version of this article appeared on July 1, 2004
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Submitted on October 17, 2003
Revised on March 11, 2004
Accepted on April 12, 2004
1 Laboratory of Parasitic Diseases, NIAID, NIH, Bethesda, Maryland 20892
* Corresponding author. E-mail address: mtouz{at}niaid.nih.gov.
Giardia lamblia is an early branching protist that possesses peripheral vacuoles (PVs) with characteristics of lysosome-like organelles, located underneath the plasma membrane. In more evolved cells, lysosomal protein trafficking is achieved by cargo recognition involving adaptor protein complexes (AP) that recognize specific amino acid sequences (tyrosine and/or dileucine motifs) within the cytoplasmic tail of membrane proteins. Previously, we reported that Giardia has a tyrosine-based sorting system, which mediates the targeting of a membrane-associated cysteine protease (ESCP) to the PVs. Here, we show that Giardia adaptor protein complex 1 (AP1) mediates the transport of ESCP and the soluble acid phosphatase (AcPh) to the PVs. By using the yeast-two hybrid assay we found that the ESCP tyrosine-based motif interacts specifically with the medium subunit of AP1 (Giµa). HA-tagged Giµa colocalizes with ESCP and AcPh, and coimmunoprecipitates with clathrin suggesting that protein trafficking toward the PVs is clathrin-adaptin dependent. Targeted disruption of Giµa results in mislocalization of ESCP and AcPh but not of variant-specific surface proteins (VSPs). Our results suggest that, unlike mammalian cells, only AP1 is involved in anterograde protein trafficking to the PVs in Giardia. Moreover, even though Giardia trophozoites lack a morphologically discernible Golgi apparatus, the presence of a clathrin-adaptor system suggests that this parasite possess a primitive secretory organelle capable of sorting proteins similar to that of more evolved cells.
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