J Biol Chem, Vol. 273, Issue 12, 6650-6655, March 20, 1998

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

Joke Ouwendijk, Wilma J. M. Peters, Rinske A. van de Vorstenbosch, Leo A. Ginsel, Hassan Y. Naim^§, and Jack A. M. Fransen

From the  Department of Cell Biology and Histology, University of Nijmegen, P. O. Box 9101, 6500 HB Nijmegen, The Netherlands and ^§ Department of Physiological Chemistry, Hannover School of Veterinary Medicine, Bünteweg 17, D-30559 Hannover, Germany

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 LPH and the 100-kDa profragment (LPH). Since LPH is not transport-competent when it is expressed separately from LPH in COS-1 cells, it was suggested that LPH functions as an intramolecular chaperone. What happens to LPH after cleavage is still unclear.

To analyze and localize LPH 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 LPH region. Wild type and tagged proteins were processed at similar rates, and both cleaved LPH 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. LPH alone, however, could not be recovered by these antibodies. Our data suggest that LPH is degraded immediately after cleavage.