Biosynthesis and intracellular transport of rat liver 5'-nucleotidase

To investigate biosynthesis and intracellular transport of 5'-nucleotidase, we purified this enzyme from rat liver and prepared antibodies. In immunoblot analysis, 5'-nucleotidase from the plasmalemmal, Golgi, and tritosomal fractions migrated as a single band of 72 kDa. The immunoreactive 68-kDa band was detected in the rough microsomal fraction and only the 72-kDa polypeptide contained sialic acids. The single polypeptide of 61 kDa immunoprecipitated from the translation products with the membrane-bound polysomal RNAs by the cell-free system converted to the 68-kDa form associated with membranes, when translated in the presence of microsomal vesicles. 5'-Nucleotidase from cultured primary hepatocytes pulse-labeled with [35S]methionine for 20 min migrated as a 68-kDa polypeptide. Within 45 min of chase, the 68-kDa form was converted to the 72-kDa form. Upon treatment with tunicamycin, a new immunoreactive polypeptide of 59 kDa was obtained. These results suggest that 5'-nucleotidase is translated on the membrane-bound polysomes as a 61-kDa precursor and that the cleavage of the 2-kDa signal peptide and the core glycosylation co-translationally convert the precursor to the 68-kDa form, which is subsequently processed in the Golgi complex to the 72-kDa form.