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J. Biol. Chem., Vol. 277, Issue 52, 50557-50563, December 27, 2002
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From the § Catedra de Bioquímica y
Biología Molecular, Facultad de Ciencias Médicas,
Universidad Nacional de Córdoba, CP 5000 Córdoba, Argentina
and the Giardia
lamblia is a flagellate protozoan that infects humans and other
mammals and the most frequently isolated intestinal parasite worldwide.
Giardia trophozoites undergo essential biological changes
to survive outside the intestine of their host by differentiating into
infective cysts. Cyst formation, or encystation, is considered one of
the most primitive adaptive responses developed by eukaryotes early in
evolution and crucial for the transmission of the parasite among
susceptible hosts. During this process, proteins that will assemble
into the extracellular cyst wall (CWP1 and CWP2) are transported to the
cell surface within encystation-specific secretory vesicles (ESVs) by a
developmentally regulated secretory pathway. Cyst wall proteins (CWPs)
are maintained as a dense material inside the ESVs, but after
exocytosis, they form the fibrillar matrix of the cyst wall. Little is
known about the molecular mechanisms involved in granule biogenesis and
discharge in Giardia, as well as the assembly of the
extracellular wall. In this work, we provide evidences that a
novel 54-kDa protein that exclusively localizes to the ESVs is induced
during encystation similar to CWPs, proteolytically processed during
granule maturation, and able to bind calcium in vitro. The
gene encoding this molecule predicts a novel protein (called gGSP for
G. lamblia
Granule-specific Protein) without
homology to any other protein reported in public databases. Nevertheless, it possesses characteristics of calcium-sequestering molecules of higher eukaryotes. Inhibition of gGSP expression abolishes
cyst wall formation, suggesting that this secretory granule protein
regulates Ca2+-dependent degranulation
of ESVs during cyst wall formation.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AF293411 and AF293409.
Identification and Characterization of a Novel
Secretory Granule Calcium-binding Protein from the Early Branching
Eukaryote Giardia lamblia*,
§,, and Laboratory of Parasitic Diseases, NIAID, National
Institutes of Health, Bethesda, Maryland 20892
*
This work was supported by the Agencia Nacional para la
Promocion de la Ciencia y la Tecnologia (ANPCYT), Fundacion Antorchas, Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), and the Howard Hughes Medical Institute.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
The on-line version of this article (available at
http://www.jbc.org) contains three supplementary figures illustrating
the purification of ESVs from encysting Giardia
trophozoites; Western blot analysis of Giardia trophozoites
showing that protein recognized by the mAb is induced during
encystation similar to CWPs; and Western blots showing that gGSP is in
a soluble form inside secretory vesicles.
¶
To whom correspondence should be addressed: Cátedra de
Bioquímica y Biología Molecular, Facultad de Ciencias
Médicas, Universidad Nacional de Córdoba, Pabellón
Argentina 2° piso, Ciudad Universitaria, CP 5000, Córdoba, Argentina. Tel./Fax: 54-351-433-3024; E-mail:
hlujan@biomed.uncor.edu.
Copyright © 2002 by The American Society for Biochemistry and Molecular Biology, Inc.
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