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1. As newly synthesized soluble & membrane glycoproteins pass though cis & medial Golgi cisternae,
most of the mannose residues are also removed from the core oligosaccharides
2. Other sugars are added sequentially by various glycosyltransferases to produce a variety of different
oligosaccharides
B. In Golgi, as in RER, sequences in which sugars are inserted into oligosaccharides is determined by spatial
arrangement of specific glycosyltransferases that contact new proteins as they pass through
1. Sialyltransferase (puts sialic acid at chain terminal position in animal cells) is found in trans end of Golgi
stack; expected if new glycoproteins were continually moving toward this part of organelle
2. In ER, a single core oligosaccharide is assembled; in Golgi complex, glycosylation steps can be quite
varied, producing carbohydrate domains of remarkable sequence diversity
3. Proteins in RER lack sugars that are normally added in medial & trans Golgi cisternae
C. Unlike N-linked oligosaccharides, whose synthesis starts in ER, those attached to proteins by O-linkages are
assembled wholly within Golgi complex
V. Vesicular transport within Golgi; how do materials move through Golgi? —> 2 contrasting theories
A. Cisternal maturation model (up to mid-1980s) – it was accepted that cisternae were transient structures; form
at cis face by ER/ERGIC vesicle fusion, travel to trans face & altered along the way
1. Cisternae mature & change in composition as they move through Golgi complex; each cisterna matures
into next cisterna along stack (origin of name)
2. Each cisterna was thought to physically move from the cis to the trans end of the stack, changing in
composition as it progressed
B. New model favored (mid-1980s until late-1990s) – cisternae of Golgi stack remain in place as stable
compartments held together by protein scaffold; known as the Vesicular Transport Model
1. Cargo (secretory, lysosomal, membrane proteins) is shuttled through Golgi stack from CGN to TGN in
vesicles that bud from one compartment & fuse with neighboring one farther along stack
VI. Acceptance of Vesicular Transport Model based largely on the following observations:
A. Each of the various Golgi cisternae of stack has distinct resident enzyme population; how could various
cisternae have such different properties if each gave rise to next in line as stated by other model?
B. Large numbers of vesicles are seen in electron micrographs to bud from rims of Golgi cisternae - James
Rothman, et al. (Stanford, 1983)
1. Using cell-free preparations of Golgi membranes, they showed that transport vesicles could bud from
one Golgi cisterna & fuse with another Golgi cisterna in vitro
2. Formed basis for hypothesis suggesting that inside cell, cargo-bearing vesicles budded from cis-cisternae
& fused with cisternae derived from a more trans position in stack
VII. Both models still have proponents, but consensus has shifted in past few years back to cisternal maturation
model; several major reasons summarized below:
A. Cisternal maturation (CM) model envisions a highly dynamic Golgi complex in which major elements of
organelle, the cisternae, are continually being formed at the cis face & dispersed at the trans face
1. According to this view, the very existence of the Golgi complex itself depends on the continual influx of
transport carriers from the ER & ERGIC
2. As CM model says, when transport carrier formation from ER is blocked either by cell treatment with
specific drugs or use of temperature-sensitive mutants, Golgi complex simply disappears
3. When the drugs are removed or the mutant cells are returned to the permissive temperature, the Golgi
complex rapidly reassembles as ER-to-Golgi transport is renewed
B. New evidence for CM model - certain materials that are produced in ER & then travel through Golgi complex
can be shown to stay in Golgi cisternae & never appear within Golgi-associated transport vesicles
1. Example: fibroblast studies – large complexes of procollagen molecules (extracellular collagen
precursors) move from cis cisternae to trans cisternae without ever leaving the cisternal lumen
C. Until mid-1990s, it was assumed that transport vesicles always moved in forward (anterograde) direction,
from cis origin to trans destination, but new evidence says that……
1. Some move in backward (retrograde) direction from trans donor to cis acceptor membrane