The membrane topography of ecto-5'-nucleotidase in rat hepatocytes

The transmembrane topography of the rat hepatocyte ectoenzyme 5'-nucleotidase was studied by the use of glycoprotein labelling and limited-proteolysis techniques. Comparison, by one-dimensional peptide mapping, of enzyme iodinated from outside the cell with that iodinated in the solubilized state showed that no additional iodination sites were revealed on solubilization. Incubation of newly synthesized enzyme in a microsomal membrane fraction with proteinase showed that the entire molecule of 5'-nucleotidase was protected from proteolysis. These data suggest that little, if any, of the 5'-nucleotidase molecule is present on the cytoplasmic side of the plasma membrane. No evidence was found for a previously proposed interaction between 5'-nucleotidase and actin, although the ability of preparations of 5'-nucleotidase to prevent inhibition of deoxyribonuclease I by actin was explained by minute traces of ATPase activity. Comparison of peptide maps of enzyme labelled by iodination or by methods specific for carbohydrate showed that in both cases predominantly one section of the molecule was labelled. It is proposed that the enzyme is a short-stalked integral membrane protein without a cytoplasmic domain in which about one-third of the molecule forms the accessible molecular surface.