The phosphorylation state of an autoregulatory domain controls PACS-1-directed protein traffic

Abstract

PACS‐1 is a cytosolic sorting protein that directs the localization of membrane proteins in the trans‐Golgi network (TGN)/endosomal system. PACS‐1 connects the clathrin adaptor AP‐1 to acidic cluster sorting motifs contained in the cytoplasmic domain of cargo proteins such as furin, the cation‐independent mannose‐6‐phosphate receptor and in viral proteins such as human immunodeficiency virus type 1 Nef. Here we show that an acidic cluster on PACS‐1, which is highly similar to acidic cluster sorting motifs on cargo molecules, acts as an autoregulatory domain that controls PACS‐1‐directed sorting. Biochemical studies show that Ser278 adjacent to the acidic cluster is phosphorylated by CK2 and dephosphorylated by PP2A. Phosphorylation of Ser278 by CK2 or a Ser278→Asp mutation increased the interaction between PACS‐1 and cargo, whereas a Ser278→Ala substitution decreased this interaction. Moreover, the Ser278→Ala mutation yields a dominant‐negative PACS‐1 molecule that selectively blocks retrieval of PACS‐1‐regulated cargo molecules to the TGN. These results suggest that coordinated signaling events regulate transport within the TGN/endosomal system through the phosphorylation state of both cargo and the sorting machinery.