INTRACELLULAR TRAFFICKING OF FURIN IS MODULATED BY THE PHOSPHORYLATION STATE OF A CASEIN KINASE-II SITE IN ITS CYTOPLASMIC TAIL

Human furin catalyzes the proteolytic maturation of many proproteins i n the exocytic and endocytic secretory pathways by cleavage at the C-t erminal side of the consensus sequence -ArgXaaLys/ArgArg down arrow-. Both the trans-Golgi network (TGN) concentration and intracellular rou ting of furin require sequences in its 56 amino acid cytoplasmic tail, Here, we show that the furin cytoplasmic tail contains multiple traff icking signals. Localization to the TGN requires a cluster of acidic a mino acids that, together with a pair of serine residues, forms a case in kinase II (CK II) phosphorylation site, We show that CK II efficien tly phosphorylates these serines in vitro, and using a permeabilized c ell system we provide evidence that CK II is the in vivo furin kinase, Analysis by mass spectrometry shows that, in vivo, furin exists as di -, mono- and nonphosphorylated forms, Finally, employing (i) furin con structs that mimic either non-phosphorylated or phosphorylated furin a nd (ii) the phosphatase inhibitor tautomycin, we show that the phospho rylation state of the furin cytoplasmic tail modulates retrieval of th e endoprotease to the TGN, Thus, routing of furin is a two-tiered proc ess combining a set of trafficking signals comprised of the primary am ino acid sequence of the tail with its phosphorylation state.