FURIN, PC1 3, AND/OR PC6A PROCESS RABBIT, BUT NOT HUMAN, PRO-LACTASE-PHLORHIZIN HYDROLASE TO THE 180-KDA INTERMEDIATE/

Small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a large precursor (prepro-LPH) of 1926 amino acids. In the endoplasmic reticulum, prepro-LPR is split by signal protease. The resulting pro-L PH is cut to mature LPH directly (human) or via a 180-kDa intermediate (rabbit), most likely in the trans-Golgi network or in a later compar tment. Antibodies directed against different regions of rabbit pro-LPH locate the cleavage site resulting in the 180-kDa intermediate betwee n amino acid residues 79 and 286. This stretch contains the two sequen ces -Arg-Cys-Tyr-Arg(114)similar to and -Arg-Ala-Ser-Arg(191)similar t o, which are potential cleavage sites for subtilisin-like proprotein c onvertases. These sites are not conserved in human pro-LPH, By coexpre ssion in COS 7 cells of rabbit prepro-LPH and proprotein convertases ( PC1/3, PC2, PC6A, PC6B, furin), we show that furin, PC1/3, and PC6A ge nerate a processing intermediate that is immunologically indistinguish able from the one observed in vivo. Furin, PC1/3, and PC6A are all exp ressed in the small intestine as shown by a polymerase chain reaction- based approach and, more specifically, in enterocytes, as shown by in situ hybridization. These results suggest that furin, PC1/3, and/or PC 6A are responsible for the in vivo processing of rabbit pro-LPH to the 180-kDa intermediate.