Intercompartmental transport in the Golgi complex is a dissociative process: facile transfer of membrane protein between two Golgi populations.

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Abstract
The transfer of the vesicular stomatitis virus-encoded glycoprotein (G protein) between Golgi populations in fused cells is exploited here to study and to help define the compartmental organization of the Golgi stack and to characterize the mechanism of intercompartmental transport. G protein that has just received its peripheral N-acetylglucosamine in the Golgi complex of 1 cell is efficiently transferred to the Golgi complex of another cell to receive galactose (Gal). Remarkably, this transport occurs at the same rate between these 2 compartments whether they are present in the same or different Golgi populations. Therefore, a dissociative (presumably vesicular) transport step moves G protein from 1 part of the Golgi in which N-acetylglucosamine is added to another in which Gal is added. Minutes later, upon receiving Gal, the same G protein molecules are very poorly transferred to an exogenous Golgi population after cell fusion. Therefore, once this intercompartmental transfer has already taken place (before fusion), it cannot take place again (after fusion); i.e., transport across the compartment boundary in the Golgi complex that separates the sites of N-acetylglucosamine and Gal incorporation is a vectorial process. Evidently, transfers between Golgi cisternae occur by a stochastic process in which transport vesicles budding from cisternae dissociate, diffuse away, and then attach to and fuse with the appropriate target cisterna residing in the same or in a different stack, based on a biochemical pairing after a random encounter. Under these circumstances, a transported protein would almost always randomize among stacks with each intercisternal transfer; it would not progress systematically through a single stack. Altogether, these studies define 3 sequential compartments in the Golgi stack.