Proteins are sorted in the trans-Golgi network (TGN) to be transported by specific vesicles to their final destinations. Formation of exocytic transport vesicles at the TGN is regulated by trimeric G protein subunits Gαi-2 and Gαi-3, by PKC which phosphorylates MARCKS and MacMARCKS and by dynamin II. In vitro-formed vesicles were studied by electron microscopy, by immunoelectron microscopy and by immunoblotting of cargo and coat proteins. Individual vesicle types coated with adaptor proteins AP-1 or AP-3, that transport lysosomal enzymes or lysosomal membrane proteins as well as vesicles characterized by KDEL receptor and β-COP, markers of the retrograde pathway, have been identified. Exocytic vesicles may represent at least three vesicle classes which exclusively or predominantly transport soluble secretory proteins, heparansulfate proteoglycans or membrane proteins to the plasma membrane.
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