Routing and Processing of Lactase-Phlorizin Hydrolase in Transfected Caco-2 Cells

Abstract Human lactase-phlorizin hydrolase (LPH) is a digestive enzyme that is expressed in the small intestinal brush-border membrane. After terminal glycosylation in the Golgi apparatus, the 230-kDa pro-LPH is cleaved into the 160-kDa brush-border LPHβ and the 100-kDa profragment (LPHα). Since LPHβ is not transport-competent when it is expressed separately from LPHα in COS-1 cells, it was suggested that LPHα functions as an intramolecular chaperone. What happens to LPHα after cleavage is still unclear. To analyze and localize LPHα in polarized epithelial cells, wild type and tagged LPH were stably expressed in Caco-2 cells. In tagged LPH, a vesicular stomatitis virus epitope tag was inserted into the LPHα region. Wild type and tagged proteins were processed at similar rates, and both cleaved LPHβ forms were expressed at the apical cell surface. Pro-LPH was recognized by antibodies against LPH, a profragment epitope and the vesicular stomatitis virus tag. LPHα alone, however, could not be recovered by these antibodies. Our data suggest that LPHα is degraded immediately after cleavage.