Cargoes move from cis to trans-Golgi compartments and concentrate in the TGN before exiting

Abstract

The classical models of intra-Golgi transport envision a movement of cargoes from cis- to trans-Golgi, followed by their sorting at the trans-Golgi network (TGN). During this vectorial transport, the cargoes are processed by sequentially acting glycosylation enzymes. A number of studies challenged the vectorial transport model and proposed alternative transport routes bypassing either directional transport or the TGN. We have re-visited intra-Golgi transport using varied cargo synchronization protocols, employing both imaging and biochemical methods. We find that cargoes move vectorially across the Golgi stack and reach the TGN. Cell type-dependent variations in transport kinetics and the limited resolution of fluorescence microscopy could have influenced earlier discrepant interpretations. Further, we find that exit from TGN is a rate-limiting step leading to the accumulation of cargoes there before their monoexponential exit. These findings support an intriguing model of intra-Golgi transport, which involves classical vectorial transport across the Golgi stack followed by a non-maturation-based transport from the stack to the TGN.