RESUMEN
We report the functional characterization of a new UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T) (EC 2.4.1.41) from the human disease-causing parasite, Toxoplasma gondii. This glycosyltransferase is denoted as T. gondii ppGalNAc-T3. These enzymes are responsible for the initial step of mucin-type O-glycosylation: the transfer of GalNAc from the UDP-GalNAc nucleotide sugar donor onto a peptide acceptor. Following an in silico analysis of the publicly available T. gondii DNA database, we used molecular biology approaches to identify and isolate the cDNA encoding this enzyme. The resulting type II membrane protein contains N-terminal cytoplasmic, transmembrane, and C-terminal lumenal domains. Conceptual translation of the cDNA sequence also reveals a stem region and the presence of several important sequence motifs. When the recombinant construct was expressed in stably transfected Drosophila melanogaster S2 cells, the purified protein exhibited glycosyltransferase activity in vitro against glycopeptide, but not "naked" peptide, acceptors. In addition, using reverse transcriptase-PCR, T. gondii ppGalNAc-T3 mRNA was equivalently expressed during the tachyzoite and bradyzoite developmental stages. The identification of T. gondii ppGalNAc-T3 as a functional "follow-up" glycopeptide glycosyltransferase further confirms that this human parasite has its own enzymatic O-glycosylation machinery.
Asunto(s)
N-Acetilgalactosaminiltransferasas/química , N-Acetilgalactosaminiltransferasas/metabolismo , Análisis de Secuencia de Proteína , Toxoplasma/química , Toxoplasma/enzimología , Secuencias de Aminoácidos , Secuencia de Aminoácidos , Animales , Clonación Molecular , Drosophila melanogaster/química , Drosophila melanogaster/genética , Drosophila melanogaster/metabolismo , Activación Enzimática , Datos de Secuencia Molecular , N-Acetilgalactosaminiltransferasas/genética , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Homología de Secuencia de Aminoácido , Toxoplasma/genética , Polipéptido N-AcetilgalactosaminiltransferasaRESUMEN
The initiation of mucin-type O-glycosylation is catalysed by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (EC 2.4.1.41). These enzymes are responsible for the transfer of N-acetylgalactosamine from the nucleotide sugar donor, UDP-GalNAc, to the hydroxyl group on specific serine or threonine residues in acceptor proteins. By screening a Toxoplasma gondii cDNA library, three distinct isoforms of the ppGalNAc-T gene family were cloned. Two additional isoforms were identified and partially cloned following analysis of the T. gondii genome sequence database. All of the cloned and identified ppGalNAc-T's are type II membrane proteins that share up to 50% amino acid sequence identity within the conserved catalytic domain. They each contain an N-terminal cytoplasmic domain, a hydrophobic transmembrane domain, and a lumenal domain; the latter consists of stem, catalytic, and lectin-like domains. Moreover, each of this ppGalNAc-T's contains important sequence motifs that are typical for this class of glycosyltransferases. These include a glycosyltransferase 1 motif containing the DXH sequence, a Gal/GalNAc-T motif, and the CLD and QXW sequence motifs located in alpha-, beta-, and gamma-repeats present within the lectin-like domain. The coding regions of T. gondii ppGalNAc-T1, -T2, and -T3 reside in multiple exons ranging in number from 6 to 10. Our results demonstrate that mucin-type O-glycosylation in T. gondii is catalysed by a multimember gene family, which is evolutionarily conserved from single-celled eukaryotes through nematodes and insects up to mammals. Taken together, this information creates the basis for future studies of the function of the ppGalNAc-T gene family in the pathobiology of this apicomplexan parasite.
Asunto(s)
N-Acetilgalactosaminiltransferasas/metabolismo , Toxoplasma/enzimología , Uridina Difosfato N-Acetilgalactosamina/metabolismo , Secuencia de Aminoácidos , Animales , Secuencia de Bases , ADN Complementario/genética , ADN Protozoario/genética , Genes Protozoarios , Glicosilación , Datos de Secuencia Molecular , N-Acetilgalactosaminiltransferasas/genética , Filogenia , Alineación de Secuencia , Toxoplasma/genética , Uridina Difosfato N-Acetilgalactosamina/genética , Polipéptido N-AcetilgalactosaminiltransferasaRESUMEN
We report the cloning, expression, and characterization of the first UDP-GalNAc:polypetide N-acetylgalactosaminyltransferase (ppGalNAc-T) from the human disease-causing parasite, Toxoplasma gondii. This enzyme is also the first characterized ppGalNAc-T of protozoan origin. This type of enzyme catalyzes the initial step of mucin-type O-glycosylation, that is, the transfer of GalNAc in O-glycosidic linkage to serine and threonine residues in polypeptides. We used polymerase chain reaction amplification with degenerate primers and hybridization screening of a T. gondii cDNA library to identify this enzyme. The resulting 84-kDa type II membrane protein contains a 49-amino acid N-terminal cytoplasmic domain, a 22-amino acid hydrophobic transmembrane domain, and a 680-amino acid C-terminal lumenal domain. Conceptual translation of the cDNA sequence reveals a relatively long (i.e. 135 amino acids) stem region and the presence of several important sequence motifs. The latter include a glycosyltransferase 1 (GT1) motif containing a DXH sequence, a Gal/GalNAc-T motif, and a region homologous to ricin lectin. Northern blot analysis identified a single 5.5-kb ppGalNAc-T transcript. Comparison of the cDNA and genomic DNA sequences reveals that this transferase is encoded by 10 exons in a 10 kb region. When the recombinant construct was expressed in stably transfected Drosophila melanogaster S2 cells, the purified protein exhibited transferase activity in vitro. The identification of this enzyme in T. gondii demonstrates that this human parasite has its own enzymatic machinery for the O-glycosylation of toxoplasmal proteins.