Apical secretion of hepatitis B surface antigen from transfected Madin-Darby canine kidney cells.

Hepatitis B surface antigen (HBsAg), the major envelope component of human hepatitis B virus, during infection drives the assembly and basolateral secretion from hepatocytes of both virions and subviral lipoprotein particles into the bloodstream. We studied the sorting behavior of HBsAg in the heterologous epithelial Madin-Darby canine kidney cells permanently transformed with the hepatitis B virus S gene. These cells, forming tightly packed monolayers in permeable supports, secreted HBsAg apically through a mechanism not involving transcytosis. This suggests that molecular features acting as apical addressing information, seemingly unfunctional or less efficiently used by the exocytic machinery of hepatocytes, could be contained in short hydrophilic regions of HBsAg. Lipids also could play a role in this asymmetric sorting because HBsAg is known to be secreted by forming macromolecular lipoprotein complexes rather than as a soluble protein. Together with available data, our results would imply not only the existence of tissue-specific variations in handling constitutively secreted proteins but also that these variations are strikingly dependent on the kind of protein examined. On the other hand, pulse-chase experiments with tunicamycin showed that the expression of apical information in HBsAg particles does not require N-linked glycosylation, contrasting with the known gp80 Madin-Darby canine kidney-endogenous apical secretory marker. This is the first experimental evidence that carbohydrate moieties in secretory proteins do not hold domain-specific sorting signals, a fact previously shown exclusively for membrane proteins. Thus, HBsAg provides a novel model system for the analysis of the molecular mechanisms of constitutive apical secretion.