The mammalian Golgi complex, a central hub of both anterograde and retrograde trafficking, is a ribbon of stacked cisterna with biochemically distinct compartments (reviewed in Glick and Nakano, 2009; Szul and Sztul, 2011). Anterograde cargo from the ERGIC and ER is received at the cis-Golgi, trafficked through the medial- and trans-Golgi and released through the trans-Golgi network (TGN) to the endolysosomal system and the plasma membrane. Although still under debate, current models of Golgi trafficking favour the cisternal maturation model, where anterograde cargo remain associated with their original lipid membrane during transit through the Golgi and are exposed to sequential waves of processing enzymes by the retrograde movement of Golgi resident proteins. In this way, cis-cisterna mature to medial- and trans-cisterna as the early acting Golgi enzymes are replaced by later acting ones (reviewed in Pelham, 2001; Storrie, 2005; Glick and Nakano, 2009; Szul and Sztul, 2011). More recently. a kiss-and-run (KAR) model for intra-Golgi trafficking has been proposed, which marries aspects of the cisternal maturation model with a diffusion model of transport (reviewed in Mironov et al, 2103).
Like the anterograde ERGIC-to Golgi transport step, intra-Golgi trafficking between the cisterna appears to be COPI-dependent (Storrie and Nilsson, 2002; Szul and Sztul, 2011). Numerous snares and tethering complexes contribute to the targeting and fusion events that are required to maintain the specificity and directionality of these trafficking events (reviewed in Chia and Gleeson, 2014). Golgi tethers include long coiled coiled proteins like the Golgins, as well as multisubunit tethers like the COG complex. These tethers make numerous interactions with other components of the secretory system including RABs, SNAREs, motor and coat proteins as well as components of the cytoskeleton (reviewed in Munro, 2011; Willet et al, 2013).
Retrograde traffic from the cis-Golgi back to the ERGIC and ER depends on both the COPI-dependent pathway, which appears to be important for recyling of KDEL receptors, and a more recently described COPI-independent pathway that relies on RAB6 (reviewed in Szul and Sztul, 2011; Heffernan and Simpson, 2014). RAB6 and RAB9 also play roles at the TGN side of the Golgi, where they are implicated in the docking of vesicles derived from the endolysosomal system and the plasma membrane (reviewed in Pfeffer, 2011)