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First Class Lecture notes Dynamic Cell Module
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Golgi apparatus lecture notes
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GOLGI APPARATUS & ENDOCYTIC PATHWAYGolgi organisation
Golgi organised into stacks
(cisternae) which have
different sets of enzymes
allowing modification of
proteins as they pass through
the Golgi apparatus
These are two extreme
models of how proteins traffic
through the Golgi. Cisternal maturation model and inter-cisternal vesicle
transport model. Evidence for both.
Vesicular transport model: static compartments, vesicular delivery of
material to compartment, some of which is packaged into vesicles to be
delivered to the next compartment, fusion, etc. Trafficking between
compartments in this anterograde way. Retrograde transport required to
return proteins to previous compartments.
Cisternal maturation model: rather than trafficking between cisternae,
there is cisternal maturation. This model suggests that maturation is
driven purely by retrograde trafficking. If the cisterna has a mixture of
molecules required to be transported to several different cisternae,
retrograde trafficking leads to maturation of this compartment from one
type to another. allows this distribution of molecules throughout Golgi
apparatus.
The cisternal maturation model is probably more widely accepted.
There are other models that incorprorate features of both of these models
and include tubular connections between cisternae.
Models for Golgi traffic: a critical assessment.
Abstract
A variety of secretory cargoes move through the Golgi, but the pathways and mechanisms of
this traffic are still being debated. Here, we evaluate the strengths and weaknesses of five
current models for Golgi traffic: (1) anterograde vesicular transport between stable
compartments, (2) cisternal progression/maturation, (3) cisternal progression/maturation with
heterotypic tubular transport, (4) rapid partitioning in a mixed Golgi, and (5) stable
compartments as cisternal progenitors. Each model is assessed for its ability to explain a set
of key observations encompassing multiple cell types. No single model can easily explain all
of the observations from diverse organisms. However, we propose that cisternal
progression/maturation is the best candidate for a conserved core mechanism of Golgi traffic,
and that some cells elaborate this core mechanism by means of heterotypic tubular transport
between cisternae.
CONCLUSIONS
The models described here differ in fundamental ways, but they all offer insights into
Golgi traffic. Emerging technologies will help us to distinguish between the various
proposals (Rothman 2010). Meanwhile, we can offer some tentative conclusions.
A crucial point is that ideas about the Golgi are necessarily constrained by
microscopy. Our models must fit with what we see in different cell types. By this
, criterion, cisternal progression/maturation is still the best candidate for a broadly
conserved mechanism that operates in most eukaryotes. This model provides the most
compelling explanation for key findings from mammalian, fungal, plant, and protist
cells. An illustration is given in Figure 6, which reproduces previously published
images of Golgi stacks in the alga Scherffelia dubia. Scale-forming algae can serve as
a “reality check” for models of Golgi traffic (Becker et al. 1995). Figure 6A shows a
thin-section electron micrograph of S. dubia (Perasso et al. 2000). Two Golgi stacks
are visible, but they are separated by the nucleus, strongly suggesting that each stack
operates as an independent trafficking device. Figure 6B shows a tomographic slice
from a 3D reconstruction of a rapidly frozen S. dubia cell. The excellent preservation
reveals that Golgi cisternae are separate compartments that are relatively smooth and
uninterrupted. This Golgi stack is surrounded by vesicles, presumably of the COPI
variety, but peri-Golgi vesicles in algae do not contain scales (Melkonian et al. 1991).
Images of this type have long been viewed as support for cisternal
progression/maturation (Becker et al. 1995), and the evidence remains persuasive.
At the same time, a basic cisternal progression/maturation model is unable to explain
all of the data. There is growing evidence that heterotypic tubular connections are
important for Golgi traffic in mammalian cells. Such connections may play equally
important roles in other cell types, and in microsporidia, they may have evolved as the
dominant mode of Golgi traffic. We therefore propose Model 3 (Cisternal
Progression/Maturation with Heterotypic Tubular Transport) as a working hypothesis
to guide future experimentation. Among the central issues to be explored are the
contents and directionality of COPI vesicles (Rabouille and Klumperman 2005;
Rothman 2010), the possible existence of a long-lived TGN or post-TGN
compartment (Glick and Nakano 2009), and the role of Rab GTPases in Golgi
dynamics (Pfeffer 2010).
Sorting from the trans-Golgi network (TGN)
The TGN is another important
‘sorting station’ where proteins (and
lipids) are packaged into different
vesicles. Signals for sorting for
regulated secretion are not very well
defined.
Mannose 6-phosphate receptor
packaged into vesicles and targeted
to endosomes then lysosomes,
whereas other molecules in trans-
Golgi network are targeted
constitutively to plasma membrane
and others are packaged into regulated secretory vesicles that will only
fuse with the PM upon certain stimulus receival.
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