Differential localization of coatomer complex isoforms within the Golgi apparatus

Abstract Coatomer, the coat protein of coat protein complex (COP)I-vesicles, is a soluble protein complex made up of seven subunits, α-, β-, β′-, γ-, δ-, e-, and ζ-COP. Higher eukaryotes have two paralogous versions of the γ- and ζ- subunits, termed γ1- and γ2-COP and ζ1- and ζ2-COP. Different combinations of these subunits are known to exist within coatomer complexes, and γ1/ζ1-, γ1/ζ2-, and γ2/ζ1-COP represent the major coatomer populations in mammals. The role of COPI vesicles in the early secretory pathway is the subject of considerable debate. To help to resolve this discussion, we used quantitative immunoelectron microscopy and found that significant localization differences for COPI-isoforms do exist, with a preference for γ1ζ1- and γ1ζ2-coatomer in the early Golgi apparatus and γ2ζ1-coatomer in the late Golgi apparatus. These differences suggest distinct functions for coatomer isoforms in a manner similar to clathrin/adaptor vesicles, where different adaptor proteins serve particular transport routes.