GRE Subject Test: Biochemistry, Cell, and Molecular Biology › Help with Vesicle Transport
Botulinum toxin is a neurotoxin that causes paralysis of the muscles. This is accomplished by cleavage of SNARE proteins contained within the presynaptic compartment of the neuron. Given this information, which of the following best describes how botulinum toxin causes paralysis?
Cleavage of SNAREs inhibits vesicles containing neurotransmitters from fusing to the membrane and stimulating the post-synaptic muscle
Cleavage of SNAREs disrupts the propagation of the action potential from the axon hillock to the presynaptic membrane
The toxin prevents the SNAREs from stimulating proper synthesis of neurotransmitters in the neuron
The toxin is globally toxic and the organism is paralyzed as the tissue becomes necrotic
Disruption of SNAREs reverses transport of vesicles to a retrograde direction, taking them away from the muscle and towards the cell soma
This requires knowing that SNARE proteins are required for proper vesicle fusion to the membrane, thereby permitting exocytosis of neurotransmitters into the synaptic cleft and activating the next target; muscle in this case. Paralysis comes because the muscle is not receiving any input once the toxin has cleaved/destroyed the SNARE proteins.
Which of the following motor proteins carries vesicular cargo along microtubules exclusively towards the microtubule organizing center (MTOC)?
Dynein
Kinesin
Actin
Myelin
Microfilament
Actin (microfilaments) is a cytoskeletal component, and myelin is an axon wrapping component; not molecular motors. Kinesin is a motor that moves in the plus-end direction, away from the MTOC. Dynein is the correct answer; it moves in the minus-end direction towards the MTOC.
Which of the following proteins/structures are involved in the mechanism of vesicular transport?
I. Actin microfilament cytoskeleton
II. Intermediate filament cytoskeleton
III. Kinesin
IV. Microtubule cytoskeleton
I, III, and IV
III and IV
I and II
II only
Both the actin microfilament cytoskeleton and the microtubule cytoskeleton serve important functions in vesicular transport. They serve as the structures upon which motor proteins move, essentially providing a directional tract for vesicular transport. Motor proteins, such as kinesin, associate with vesicles and bring them from one area of the cell to the other along the directional filaments.
The intermediate filament cytoskeleton lacks the polarity displayed by actin microfilaments and microtubules, making it not very useful for vesicular transport.
Which of the following portions of the cytoskeleton are used extensively for vesicular transport?
I. Actin
II. Intermediate filaments
III. Microtubules
I and III
I, II, and III
III only
I and II
Actin and microtubules have similar chemical properties. They maintain a nucleotide gradient (ADP/ATP for actin and GDP/GTP for microtubules) across their structures and have two distinct polarized ends. Intermediate filaments do not have either of these characteristics. For that reason, motor proteins associate with actin and microtubules as opposed to intermediate filaments. The polarity of the microtubules and actin allow the motor proteins to become oriented, transporting cargo in a particular direction along the structure. These motor proteins (such as myosin, dynein, and kinesin) are essential for vesicular transport.
SNARE (soluble NSF attachment protein receptor) proteins are crucial molecular mediators of vesicular exocytosis. SNAREs require calcium to mediate exocytosis; namely, one protein component of the SNARE complex interacts with synaptotagmin in a calcium dependent fashion. Which of the following answers lists the SNARE proteins that interacts with synaptotagmin?
Syntaxin
Synaptobrevin
SNAP-25
Sec1
Exo70
Synaptotagmin is a calcium sensor that is associated with the vesicle to be exocytosed. In a high calcium environment, synaptotagmin becomes activated and interacts with syntaxin, a SNARE protein docked in the membrane from which the vesicle will be exocytosed. This interaction permits selective exocytosis during processes such as neurotransmission when there is a large calcium influx, indicating a message must be relayed to the next cell.
Which of the following protein coats would most likely be seen on a vesicle directed towards the plasma membrane?
Clathrin
COPI
COPII
None of the answers
Clathrin coats are often seen trafficking vesicles from the Golgi apparatus to the plasma membrane. Clathrin protein is used to facilitate membrane invagination and vesicle formation, as well as direct vesicle release.
COPI coats are seen in vesicles headed from the Golgi apparatus back to the endoplasmic reticulum. COPII coats are seen in vesicles headed towards the Golgi apparatus from the endoplasmic reticulum.