Trypanosoma cruzi Calreticulin Is a Lectin That Binds Monoglucosylated Oligosaccharides but Not Protein Moieties of Glycoproteins

CARLOS LABRIOLA,*† JUAN J. CAZZULO,† AND ARMANDO J. PARODI*‡

MOLECULAR BIOLOGY OF THE CELL

AMER SOC CELL BIOLOGY

Lugar: Unitted States; Año: 1999 vol. 10 p. 1381 - 1392

1059-1524

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 oligosaccharideprocessing present in the endoplasmic reticulum of higher eukaryotes, itdisplays UDP-Glc:glycoprotein glucosyltransferase and glucosidase II activities. It isherewith reported that this protozoan also expresses a calreticulin-like molecule, thethird component of the quality control of glycoprotein folding. No calnexin-encodinggene was detected. Recombinant T. cruzi calreticulin specifically recognized free monoglucosylatedhigh-mannose-type oligosaccharides. Addition of anti-calreticulin serum toextracts obtained from cells pulse–chased with [35S]Met plus [35S]Cys immunoprecipitatedtwo proteins that were identified as calreticulin and the lysosomal proteinasecruzipain (a major soluble glycoprotein). The latter but not the former protein disappearedfrom immunoprecipitates upon chasing cells. Contrary to what happens inmammalian cells, addition of the glucosidase II inhibitor 1-deoxynojirimycin promotedcalreticulin– cruzipain interaction. This result is consistent with the known pathway ofprotein N-glycosylation and oligosaccharide processing occurring in T. cruzi. A treatmentof the calreticulin-cruzipain complexes with endo-b-N-acetylglucosaminidase H eitherbefore or after addition of anti-calreticulin serum completely disrupted calreticulin–cruzipain interaction. In addition, mature monoglucosylated but not unglucosylatedcruzipain isolated from lysosomes was found to interact with recombinant calreticulin. Itwas concluded that the quality control of glycoprotein folding appeared early in evolution,and that T. cruzi calreticulin binds monoglucosylated oligosaccharides but not theprotein moiety of cruzipain. Furthermore, evidence is presented indicating that glucosyltransferaseglucosylated cruzipain at its last folding stages.