DISPOSITION OF THE CARBOXY-TERMINUS TAIL OF RABBIT LACTASE-PHLORHIZINHYDROLASE ELUCIDATED BY PHOSPHORYLATION WITH PROTEIN-KINASE-A IN-VITRO AND IN TISSUE-CULTURE

The intracellular disposition of the carboxy-terminus tail of rabbit l actase-phlorizin hydrolase (LPH) is demonstrated, using a specific pho sphorylation of Ser(1916) by protein kinase A (PKA). This phosphorylat ion is shown to occur not only in vitro (with pare LPH and pure cataly tic subunit of PKA), but also in an organ culture of the small intesti ne, Cholera toxin, which is known to act in vivo on the membranes of t he small intestine, with severe clinical consequences, and to elevate the intracellular cyclic AMP of enterocytes, is shown to enhance signi ficantly the phosphorylation ofLPH in intact cells grown as an organ c ulture. These findings establish the cytosolic orientation of the carb oxy-terminus tail of LPH in situ, and raise the possibility that the t ail itself and its phosphorylation by PKA may have a physioiogical or physiopathological significance.