Membrane and vesicular trafficking in the early secretory pathway are mediated by non-Clathrin COP (coat protein) I-coated vesicles. COPI-coated vesicles mediate retrograde transport from the Golgi back to the ER and intra-Golgi transport. The cytosolic precursor of the COPI coat, the heptameric coatomer complex, is composed of two subcomplexes. The first consists of the COPB, COPG, COPD and COPZ1 subunits (also known as beta-COP, gamma -COP, delta-COP and ?-1 COP, respectively), which are distantly homologous to AP Clathrin adaptor subunits. The second consists of the COPA, COPP and COPE subunits (also known as alpha-COP, COPP and E-COP, respectively).