Compartmentation of the Golgi complex: brefeldin-A distinguishes trans- Golgi cisternae from the trans-Golgi network

Unlike the cis-, medial, and trans-Golgi cisternae, the TGN functions to sort proteins into distinct classes of transport vesicles, bound for prelysosomes (10), secretory storage granules (44) , and specific domains of the plasma membrane (14, 36) . The TGN is the site to which man6P receptors recycle (10, 17, 26, 35) , and where viral glycoproteins accumulate at 20°C (24, 41). The TGN is also a much more extended and tubular structure than the other Golgi cisternae, and thus, it is distinguished by its unique morphological characteristics. Despite numerous distinctions between the TGN and other Golgi cisternae, it is likely that the TGN is nevertheless, an integral part of the Golgi complex. Grifliths et al. (25) have shown that the TGN and more proximal Golgi cisternae are highly interrelated structures. At 20°C, the TGN increases in size, concomitant with a decrease in size of preceding Golgi compartments (25) . In addition, these workers have calculated that 12% of the TGN surface area is comprised of flattened cisternae which are morphologically indistinguishable from, and located immediately adjacent to, the other cisternae of the Golgi stack. Thus, although the TGN is functionally distinct from other Golgi cisternae (21), it is appropriate to consider it as a specialized subcompartment of the Golgi complex. An important future challenge will be to elucidate the mechanism by which the TGN gains (and maintains) its identity.