THE CARBOXYL-TERMINAL TAIL OF KINASE SPLITTING MEMBRANAL PROTEINASE MEPRIN-BETA IS INVOLVED IN ITS INTRACELLULAR TRAFFICKING

The kinase splitting membranal proteinase (KSMP), was recently shown t o be identical with the beta-subunit of meprin. Meprin is a metalloend oproteinase located in brush border membranes and composed of the two types of subunits, alpha and beta. Despite their high sequence homolog y and similar domain organization, meprin subunits are differently pro cessed during maturation; meprin ct is retained in the endoplasmic ret iculum (ER), and undergoes a proteolytic removal of the transmembrane and cytoplasmic domains, prior to its export from this organelle, In c ontrast, meprin beta retains these domains even after reaching its fin al destination in the plasma membrane. Using truncated mutants of rat meprin beta expressed in Cos-7 and human embryonic kidney (HEK) 293 ce lls, we show here that the cytoplasmic tail is indispensable for its e xit from the ER, A meprin beta mutant lacking the last 25 amino acids is shown to be transport-incompetent, although it does not contain any of the known ER retention signals, Systematic analysis of the rate of the ER to Golgi transport using a series of mutants with Ala or Pro s ubstitutions in the tail, suggests that while no specific amino acid r esidue by itself is imperative for normal intracellular trafficking of meprin beta, the insertion of a bend at a distinct position in the ta il (specifically by a Y685P mutation) suffices to retain this protein in the ER. We propose that the very length of the cytoplasmic tail, as well as its secondary structure are essential for the ER to Golgi tra nsport of meprin beta, possibly by allowing an interaction with a carg o receptor.