BIO206H5 Study Guide - Final Guide: Nuclear Export Signal, Endoplasmic Reticulum, Nuclear Localization Sequence

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18 Feb 2016
Michelle Chyjek (1001326371)
BIO206 – TUT0113
Protein Targeting Worksheet
1. There are several things required for successful protein sorting. First of all, signal
sequences are required to determine the correct destination of the protein. These are specific
amino acid sequences that are part of the protein that can be removed at the final destination.
In addition to these sequences, the protein must be transported along the correct pathway
(nuclear, secretory, or translocation) via signal sequence receptors. The receptors assist in
directing the protein towards the correct organelle or compartment. There are three types of
transport mechanisms requires for successful protein sorting: gated nuclear pores, protein
translocators, and transport vesicles. These mechanism assist the protein in traveling towards
its destination with high specificity. Energy is also required to move the protein throughout the
2. In order to perform their function properly, sorting signals should carry a slight positive or
negative charge. This helps them assist the proteins in reaching their destination more
efficiently as then can travel through an electrical field into the desired organelle. The sorting
signals should also be able to determine retention versus “onward traffic”. This ensures that
the protein knows where to stop, in which organelle and for which pathway. Sorting signals
should be unique to one target, be identifiable and accessible to the receptor, be unique to the
receptor, and must be able to be removed once the protein is in its final destination.
3. For large molecules and macromolecular complexes to gain entry into a nucleus through a
nuclear pore, they must display the nuclear localization signal, the NLS. This signal sequence
directs a protein from the cytosol into the nucleus. It typically consists of one or two short
sequences containing several positively charged lysines or arginines (consensus sequence
PPKKKRKV) in the amino acid sequence of the target molecule. The NLS on proteins destined
for the nucleus is recognized by cytosolic proteins called nuclear import receptor, which assist
in guiding the proteins towards and into the nucleus. The NLS can be located anywhere on the
protein as long as it is accessible to the nuclear import receptor.
4. There are many different signal sequences in proteins in the cell that can specify different
pathways for the protein. In contradiction to the NLS, there is the nuclear export signal, which
directs targeted proteins out of the nucleus. There are also many sequences, like the NLS, that
indicate the import of a protein into a specific organelle, for example, the endoplasmic
reticulum import signal, the mitochondria import signal, and the peroxisome import signal. The
last type of signal sequence is that which tags a protein to stay in a particular location, for
example, the signal sequence that codes for the retention of a protein in the lumen of the
endoplasmic reticulum.
5. All proteins that are made are first directed to the endoplasmic reticulum (ER). Most of the
proteins that enter the ER begin to be threaded across the ER membrane before the
polypeptide chain has been completely synthesized. Because they are translocated in this
fashion, no additional energy is required for their transport; the elongation of each polypeptide
provides the thrust needed to push the growing chain through the ER membrane. There are
two protein components that help guide ER signal sequences to the ER membrane: the signal-
recognition particle (SRP) and the SRP receptor (embedded in the ER membrane). The SRP
binds to the ribosome that is continuously synthesizing the ER signal sequenced protein and
slows the synthesis until it is bound at the SRP receptor. Once bound, the SRP is released, the
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