The Secretory Pathway From History to the State of the Art - Fusion of Biological Membranes and Related Problems - page 5

transferase I involved in remodeling of N -glycosylation is one of the marker
molecules for the medial -Golgi (Dunphy et al ., 1985).
The trans -compartment is composed of 3-6 superposed sacculo-
tubular elements which, in contrast to elements of the medial Golgi, do
not remain strictly parallel to each other. One of the last processing steps
before proteins exit from the Golgi complex are sialylation and tyrosine
sulfation catalyzed by the sialyltransferase (Roth et al ., 1985) and by the
Tyrosylprotein-sulfotransferase (Huttner, 1988), respectively.
The exit site of the Golgi, the trans -Golgi network (TGN), is a highly
variable structure depending on the cell type. It has been suggested that the
TGN itself is composed of functionally distinct subdomains that might be
involved in sorting of cargo proteins into different types of transport vesi-
cles destined for endosomes, lysosomes, secretory granules and the plasma
membrane (Matter and Mellman, 1994; Burgess and Kelly, 1987; Griffiths
and Simons, 1986).
3. TRANSPORT THROUGH THE SECRETORY PATHWAY
The various transport steps between the stations of the secretory
pathway require the packaging of cargo in some kind of containers that bud
from a donor compartment and fuse with an acceptor compartment. A
number of models have been proposed over the years to explain traffic flow
along the secretory pathway.The only two that are still under discussion are
the “vesicular model” and the “cisternal maturation model”.
The vesicular model-already suggested by Palade (1975)-predicts
that small vesicles carry cargo between distinct compartments. In order to
allow sequential processing of carbohydrates and sorting of cargo to the
correct destination, vesicle flow must be directional, i.e.a vesicle budding
from the ER specifically fuses with the cis -Golgi, a vesicle from the
cis-Golgi fuses with cisternae from the medial -Golgi, etc. (reviewed in:
Rothman and Wieland, 1996; Schekman and Orci, 1996).
A completely different view on the mode of intracellular trafficking is
based on the transport of molecules that may be too large in order to be
packaged into small vesicles, such as algal scales (Becker et al ., 1995) and
procollagen (Bonfanti et al ., 1998). In this case, it has been suggested that
transport occurs through progressive maturation of the Golgi cisternae
(Mironov et al ., 1997). According to this model, the cargo molecules would
be transported without leaving the lumen of a Golgi cisternae, i.e. without
been packaged into a separate transport vesicle. Exit from the ER is sug -
gested to be mediated by tubules elongating directly from the ER surface
(Bonfanti et al ., 1998).