RESUMO
As part of a general project aimed at elucidating the initiation of mucin-type O-glycosylation in helminth parasites, we have characterized a novel ppGalNAc-T (UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase) from the cestode Echinococcus granulosus (Eg-ppGalNAc-T1). A full-length cDNA was isolated from a library of the tissue-dwelling larval stage of the parasite, and found to code for a 654-amino-acid protein containing all the structural features of ppGalNAc-Ts. Functional characterization of a recombinant protein lacking the transmembrane domain showed maximal activity at 28 degrees C, in the range 6.5-7.5 pH units and in the presence of Cu2+. In addition, it transferred GalNAc to a broad range of substrate peptides, derived from human mucins and O-glycosylated parasite proteins, including acceptors containing only serine or only threonine residues. Interestingly, the C-terminal region of Eg-ppGalNAc-T1 bears a highly unusual lectin domain, considerably longer than the one from other members of the family, and including only one of the three ricin B repeats generally present in ppGalNAc-Ts. Furthermore, a search for conserved domains within the protein C-terminus identified a fragment showing similarity to a recently defined domain, specialized in the binding of organic phosphates (CYTH). The role of the lectin domain in the determination of the substrate specificity of these enzymes suggests that Eg-ppGalNAc-T1 would be involved in the glycosylation of a special type of substrate. Analysis of the tissue distribution by in situ hybridization and immunohistochemistry revealed that this transferase is expressed in the hydatid cyst wall and the subtegumental region of larval worms. Therefore it could participate in the biosynthesis of O-glycosylated parasite proteins exposed at the interface between E. granulosus and its hosts.
Assuntos
Echinococcus granulosus/enzimologia , Lectinas/química , N-Acetilgalactosaminiltransferases/química , Peptídeos/química , Sequência de Aminoácidos , Animais , Células COS/química , Células COS/metabolismo , Bovinos , Doenças dos Bovinos/enzimologia , Doenças dos Bovinos/parasitologia , Linhagem Celular , Chlorocebus aethiops , Clonagem Molecular , Cobre/fisiologia , DNA Complementar/genética , DNA de Helmintos/genética , Equinococose/enzimologia , Equinococose/veterinária , Proteínas de Helminto/biossíntese , Proteínas de Helminto/química , Proteínas de Helminto/genética , Proteínas de Helminto/fisiologia , Concentração de Íons de Hidrogênio , Lectinas/genética , Manganês/metabolismo , Dados de Sequência Molecular , N-Acetilgalactosaminiltransferases/biossíntese , N-Acetilgalactosaminiltransferases/genética , N-Acetilgalactosaminiltransferases/fisiologia , Peptídeos/genética , Estrutura Terciária de Proteína , Análise de Sequência de DNA , Especificidade por Substrato/genética , Polipeptídeo N-AcetilgalactosaminiltransferaseRESUMO
The inhibitor preparation of the UDP-N-acetylgalactosamine: GM3, N-acetylgalactosaminyltransferase (EC 2.4.1.92) (GalNAc-T) produces effects on the neurons and the glial (astrocytes) cells of the cerebrum in culture. The effect in culture is evidenced by aneuritogenesis, deficiency in the GalNAc-T activity, and decrease in the content of gangliosides, proteins, and lipids. In isolated glial cells the effect is evidenced by cytoplasm vesiculation and premature cessation of proliferation compared with control culture. The pattern of gangliosides in the inhibited culture shows a decrease in the amount of GD1a with respect to GD3; this is compatible with the notion that the effect is due to an inhibitor of the GM2 synthase. The inhibitor effects are reverted when it is eliminated after 24 or 48 hr in the culture medium.