The Carboxyl-terminal Tail of Kinase Splitting Membranal Proteinase/Meprin β Is Involved in Its Intracellular Trafficking

Abstract

The kinase splitting membranal proteinase (KSMP), was recently shown to be identical with the beta-subunit of meprin. Meprin is a metalloendoproteinase located in brush border membranes and composed of the two types of subunits, alpha and beta. Despite their high sequence homology and similar domain organization, meprin subunits are differently processed during maturation; meprin alpha is retained in the endoplasmic reticulum (ER), and undergoes a proteolytic removal of the transmembrane and cytoplasmic domains, prior to its export from this organelle. In contrast, meprin beta retains these domains even after reaching its final destination in the plasma membrane. Using truncated mutants of rat meprin beta expressed in Cos-7 and human embryonic kidney (HEK) 293 cells, we show here that the cytoplasmic tail is indispensable for its exit 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 substitutions in the tail, suggests that while no specific amino acid residue by itself is imperative for normal intracellular trafficking of meprin beta, the insertion of a bend at a distinct position in the tail (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 transport of meprin beta, possibly by allowing an interaction with a cargo receptor.