Differential localization of coatomer complex isoforms within the Golgi apparatus

Coatomer, the coat protein of coat protein complex (COP)l-vesicles, is a soluble protein complex made up of seven subunits, alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta-COP. Higher eukaryotes have two paralogous versions of the gamma- and zeta- subunits, termed gamma 1- and gamma 2-COP and zeta 1- and zeta 2-COP. Different combinations of these subunits are known to exist within coatomer complexes, and gamma 1/zeta 1-, gamma 1/zeta 2-, and gamma 2/zeta 1-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 gamma 1 zeta 1- and gamma 1 zeta 2-coatomer in the early Golgi apparatus and gamma 2 zeta 1-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.