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Vol. 10, Issue 5, 1381-1394, May 1999
and
*Instituto de Investigaciones Bioquímicas Fundación
Campomar, 1405 Buenos Aires, Argentina; and Trypanosoma cruzi is a protozoan parasite that
belongs to an early branch in evolution. Although it lacks several
features of the pathway of protein N-glycosylation and
oligosaccharide processing present in the endoplasmic reticulum of
higher eukaryotes, it displays UDP-Glc:glycoprotein glucosyltransferase
and glucosidase II activities. It is herewith reported that this
protozoan also expresses a calreticulin-like molecule, the third
component of the quality control of glycoprotein folding. No
calnexin-encoding gene was detected. Recombinant T.
cruzi calreticulin specifically recognized free
monoglucosylated high-mannose-type oligosaccharides. Addition of
anti-calreticulin serum to extracts obtained from cells pulse-chased
with [35S]Met plus [35S]Cys
immunoprecipitated two proteins that were identified as calreticulin
and the lysosomal proteinase cruzipain (a major soluble glycoprotein).
The latter but not the former protein disappeared from
immunoprecipitates upon chasing cells. Contrary to what happens in
mammalian cells, addition of the glucosidase II inhibitor
1-deoxynojirimycin promoted calreticulin-cruzipain interaction. This
result is consistent with the known pathway of protein
N-glycosylation and oligosaccharide processing occurring
in T. cruzi. A treatment of the calreticulin-cruzipain complexes with endo-Instituto de
Investigaciones Biotecnológicas, Universidad Nacional de General
San Martín, 1650 Provincia de Buenos Aires, Argentina
-N-acetylglucosaminidase H either
before or after addition of anti-calreticulin serum completely
disrupted calreticulin-cruzipain interaction. In addition, mature
monoglucosylated but not unglucosylated cruzipain isolated from
lysosomes was found to interact with recombinant calreticulin. It was
concluded that the quality control of glycoprotein folding appeared
early in evolution, and that T. cruzi calreticulin binds
monoglucosylated oligosaccharides but not the protein moiety of
cruzipain. Furthermore, evidence is presented indicating that
glucosyltransferase glucosylated cruzipain at its last folding stages.
Corresponding author. E-mail address:
aparodi{at}iib.uba.ar.
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