INTERACTION OF FURIN IN IMMATURE SECRETORY GRANULES FROM NEUROENDOCRINE CELLS WITH THE AP-1 ADAPTER COMPLEX IS MODULATED BY CASEIN KINASE-II PHOSPHORYLATION

The composition of secretory granules in neuroendocrine and endocrine cells is determined by two sorting events; the first in the trans-Golg i complex (TGN), the second in the immature secretory granule (ISG), S orting from the ISG, which may be mediated by the AP-1 type adaptor co mplex and clathrin-coated vesicles, occurs during ISG maturation, Here we show that furin, a ubiquitously expressed, TGN/endosomal membrane endoprotease, is present in the regulated pathway of neuroendocrine ce lls where it is found in ISGs, By contrast, TGN38, a membrane protein that is also routed through the TGN/endosomal system does not enter IS Gs, Furin, however, is excluded from mature secretory granules, sugges ting that the endoprotease is retrieved from the clathrin-coated ISGs, Consistent with this, we show that the furin cytoplasmic domain inter acts with AP-1, a component of the TGN/ISG-localized clathrin sorting machinery, Interaction between AP-1 and furin is dependent on phosphor ylation of the enzyme's cytoplasmic domain by casein kinase II, Finall y, in support of a requirement for the phosphorylation-dependent assoc iation of furin with AP-1, expression of furin mutants that mimic eith er the phosphorylated or unphosphorylated forms of the endoprotease in AtT-20 cells demonstrates that the integrity of the CKII sites is nec essary for removal of furin from the regulated pathway.