THE RECYCLING OF ERGIC-53 IN THE EARLY SECRETORY PATHWAY - ERGIC-53 CARRIES A CYTOSOLIC ENDOPLASMIC-RETICULUM EXIT DETERMINANT INTERACTING WITH COPII

Further investigation of the targeting of the intracellular membrane l ectin endoplasmic reticulum (ER)-Golgi intermediate compartment-53 (ER GIC-53) by site-directed mutagenesis revealed that its lumenal and tra nsmembrane domains together confer ER retention. In addition we show t hat the cytoplasmic domain is required for exit from the ER indicating that ERGIC-53 carries an ER-exit determinant. Two phenylalanines at t he C terminus are essential for ER-exit. Thus, ERGIC-53 contains deter minants for ER retention as well as anterograde transport which, in co njunction with a dilysine ER retrieval signal, control the continuous recycling of ERGIC-53 in the early secretory pathway, In vitro binding studies revealed a specific phenylalanine-dependent interaction betwe en an ERGIC-53 cytosolic tail peptide and the COPII coat component Sec 23p. These results suggest that the ER-exit of ERGIC-53 is mediated by direct interaction of its cytosolic tail with the Sec23p.Sec24p compl ex of COPII and that protein sorting at the level of the ER occurs by a mechanism similar to receptor-mediated endocytosis or Golgi to ER re trograde transport.