BIOL 4004 Lecture Notes - Lecture 1: Threonine, Digestion, Proteoglycan
7 pages26 viewsFall 2015
Course CodeBIOL 4004
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MBC5 Study Guide – Chapter 13 (Intracellular Vesicular Traffic)
Thus far, we have considered protein-targeting mechanisms that involve receptor binding
and the subsequent movement of proteins through channels, either posttranslationally or
cotranslationally. This chapter considers another way that proteins move from compartment
to compartment via membrane vesicles. It occurs between various cellular compartments as
depicted in Figure 13-3.
THE MOLECULAR MECHANISMS OF MEMBRANE TRANSPORT AND THE
MAINTENANCE OF COMPARTMENTAL DIVERSITY
Figure 13-2 gives a general overview of vesicular transport. A membrane vesicle buds from
a donor compartment and later fuses with a target compartment. As shown in Figure 13-3,
this type of vesicular transport occurs throughout the secretory pathway.
There Are Various Types of Coated Vesicles
The vesicles that bud from donor membranes are termed coated vesicles because they are
coated with particular protein molecules. The protein coats promote the budding process and
also play a role in the fusion of the vesicle with the correct target membrane. The three
types, clathrin, COPI , and COPII, are shown in Figure 13-4. We will consider where these
different types of vesicles are formed as we progress through this chapter.
The Assembly of a Clathrin Coat Drives Vesicle Formation
Clathrin proteins assemble into a cage-like structure that pulls a portion of the lipid bilayer
out of the surrounding membrane. In other words, the assembly of clathrin causes a bud to
form. This process is depicted in Figure 13-8. Another protein, called adaptin, is contained
within the budding vesicle. Adaptin promotes the formation of the clathrin coat, but only if a
cargo molecule is bound to a receptor. In this way, a bud only forms if it has some cargo to
Not All Coats form Basketlike Structures
Phosphoinositides Mark Organelles and Membrane Domains
Cytoplasmic Proteins Regulate the Pinching-off and Uncoating of Coated Vesicles
This subtitle doesn’t seem to go with the content. Later, we’ll consider how GTPases
regulate this process. In any case, this subsection explains how dynamin pinches the neck of
the bud to release the vesicle from the donor membrane.
Monomeric GTPases Control Coat Assembly
Vesicle budding and fusion is a highly regulated process. To ensure balance between
budding and fusion, the process is regulated by ARF and SarI proteins. Both types are
GTPases. The exact mechanism by which these proteins strike a balance is not entirely
understood. However, they interact directly with vesicles to exert their effects. For example,
as shown in Figure 13-13, Sar1, in its GTP bound form, can insert into budding membranes
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and promote bud formation. Presumably, the balance between GDP- and GTP-bound Sar1
helps to regulate the budding process.
Not All Transport Vesicles are Spherical
Rab Proteins Guide Vesicle Targeting
Another issue in vesicle transport is specificity. When a vesicle buds from a donor
membrane, what ensures that it will fuse with the correct target membrane? The answer is
that Rab Proteins and SNARE proteins (see below) determine this targeting. As shown in
Figure 13-14 and 13-15, Rab proteins are incorporated into vesicles. There are many types
of Rab proteins and Rab effectors. The specificity between a given Rab protein and a Rab
effector is a one way to ensure that a vesicle fuses with the correct target membrane. The
interaction of Rab protein and a Rab effector also helps promote interaction between a v-
SNARE and a t-SNARE (see below).
SNAREs Mediate Membrane Fusion
In addition to the interactions between Rab proteins and Rab-Effectors, proteins called v-
SNARES and t-SNAREs also help to promote specificity of vesicle fusion. A type of
SNARE called a v-SNARE is incorporated into a budding membrane. There are various
types of v-SNAREs and t-SNAREs; each type is relatively specific for a particular
compartment in the cell. The v-SNARE in a vesicle recognizes a t-SNARE in the target
membrane and this specificity helps to ensure that the vesicle fuses with the correct
membrane. The v-SNARE and t-SNARE interaction also drive membrane fusion.
Interacting SNAREs Need To Be Pried Apart Before They Can Function Again
SNAREs need to be recycled back to the compartment from which they originated, but first
they must be separated so that they may function again when they get back. This requires
energy, ATP, and is performed by NSF ATPases (see Fig 13-22).
Viral Fusion Proteins and SNAREs May Use Similar Fusion Mechanisms
TRANSPORT FROM THE ER THROUGH THE GOLGI APPARATUS
Transport through the stacks of membranes that constitute the Golgi follows many of the
same principles that we have already considered. In this section, we will mostly focus on
some of the unique issues associated with the Golgi.
Proteins Leave the ER in COPII-coated Transport Vesicles
Only Proteins That Are Properly Folded and Assembled Can Leave the ER
Vesicular Tubular Clusters Mediate Transport from the ER to the Golgi Apparatus
Thus far, we have considered how a vesicle can bud from a donor membrane and fuse with a
target membrane. Another thing that can happen is that vesicles can fuse with each other.
This happens to certain vesicles that bud from the ER. They fuse with each other to form
vesicular tubular clusters. Such clusters ultimately make a new compartment at the cis-end
of the Golgi (Fig 13-23).
The Retrieval Pathway to the ER Uses Sorting Signals
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