Brefeldin A Revealing the Fundamental Principles Governing Membrane Dynamics and Protein Transport - Fusion of Biological Membranes and Related Problems - page 236

“direction” that vesicles travel has been reversed does lead to a profound
change in the concepts of membrane dynamics through the ER-Golgi
system. The anterograde vesicular transport model postulates pre-existing
compartments that receive and then emit transport vesicles that carry
anterograde-bound cargo. This is conceptually the easiest model to visual-
ize, since we can think of the secretory pathway as a series of discrete boxes
with defined transport carriers moving from one to the next. In the matu-
ration model, distinct Golgi compartments do not exist as stable entities in
themselves, but rather undergo continual transformation as resident
enzymes are removed and taken backwards to a previous compartment. B.
Glick and collegues developed a mathematical model that describes how
Golgi enzymes could be concentrated in different regions of the Golgi stack
in such a maturation model (Glick et al., 1997).The central concept is a dif-
ference in affinity of different resident Golgi enzymes for the retrograde
transport machinery or more specifically, for the COPI coat. This model
accomodates recent data (using the in vitro intra-Golgi transport assay)
showing that Golgi-derived vesicles which co-fractionate with COPI
contain Golgi glycosylation enzymes rather than anterograde cargo (Love
et al., 1998). A variation of this model is that instead of carriers moving
backwards from saccule to saccule in a stepwise manner, there is one single
retrograde pathway from the trans Golgi/TGN to the cis side of the Golgi
apparatus (which would explain the need for only one t-SNARE in the
Golgi apparatus). However, the original mathematical model as described
requires stepwise retrograde transport events to maintain a distinct distri-
bution of enzymes (Glick et al., 1997).
2.2. The Three-Dimensional Structure of Intracellular Organelles and
the Concept of Membrane Transformation
Beginning in 1974, Alain Rambourg, Yves Clermont and their collab-
orators developed novel electron microscopy techniques that combined the
use of selective impregnation methods of diverse membrane systems, obser-
vation with the aid of stereoscopic techniques, and sections of different
thicknesses. These novel techniques permitted the visualisation of the three-
dimensional structure of cellular organelles (Rambourg and Clermont,
1990). A particularly useful impregnation method used by Rambourg, Cler-
mont and collegues was that of Karnovsky, which selectively contrasts all
intracellular membranes and hence allows visualisation of the entire ER-
Golgi system in one preparation. Rambourg and Clermont were able to
demonstrate in a definitive manner that the Golgi apparatus of mammalian
cells does not consist of independent structures (stacks of saccules) but
rather is one continuous entity with the appearance of a continuous ribbon