IMMUNOGLOBULIN-DERIVED POLYPEPTIDES ENTER THE REGULATED SECRETORY PATHWAY IN ATT-20 CELLS

Constitutively secreted proteins have traditionally been believed to b e excluded from the regulated secretory pathway. In this work we show that kappa light chain and Fc fragment two markers of the constitutive pathway, are present in the regulated pathway in AtT-2O cells. They c olocalize with the endogenous hormone ACTH and they exhibit stimulus-d ependent secretion. The Fc fragment, which undergoes intracellular tra nsport at the same rate as the ACTH precursor POMC, enters the forming secretory granules, however, it is partially lost during granule matu ration. These observations show that classic constitutive secretory ma rkers are not excluded from the regulated secretory pathway and that e fficient sorting for regulated secretion occurs above a background of proteins which enter the granules without sorting. (C) 1998 Federation of European Biochemical Societies.