Chapter 12: The Endomembrane System and Protein Sorting (Pt. 2)

Proteins synthesized in the rough E R must…

be directed to a variety of locations

Once a protein reaches its destination, it must…

be prevented from leaving

tag

each protein contains a specific one targeting it to a transport vesicle that will take it to the correct location

A tag may be an…

amino acid sequence, a hydrophobic domain, oligosaccharide side chain, or some other feature. (Tags can also exclude material from certain vesicles)

Tagged polypeptides encoded by nuclear genes begin with…

transcription of DNA into RNAs. •These RNAs are processed in the nucleus (becoming mRNAs) and then transported to the cytoplasm.

mRNAs associate with…in the cytoplasm and…begins

free ribosomes, translation

Shortly after translation begins…

two pathways for sorting the synthesizing polypeptide diverge

1.Pathway for polypeptides destined for the cytosol, mitochondria, chloroplast, peroxisome, and nuclear interior:

Free ribosomes synthesize these polypeptides; After translation, the polypeptide is released. It remains in the cytosol, or an organelle takes it up.

Posttranslational import

the uptake of completed polypeptides with special targeting signals into the organelle

2.Pathway for polypeptides destined for the endomembrane system or export from the cell:

The ribosome (bound ribosomes) synthesizing these polypeptides attach to the ER membrane.

cotranslational import

The polypeptide is transferred across (or for integral membrane proteins, inserted into) the ER membrane as synthesis occurs

These proteins can be directed to the…

ER, Golgi apparatus, endosomes and lysosome.

Other proteins are incorporated in the…

plasma membrane or released outside the cell.

Proteins Are Transported into Organelles by…

Three Mechanisms

Signal Sequences

Direct Proteins to the Correct Compartment

Proteins Enter the Nucleus Through…

Nuclear Pores

Proteins Unfold to….

Enter Mitochondria and Chloroplasts

Proteins Enter Peroxisomes from…

both the Cytosol and the Endoplasmic Reticulum

Proteins Enter the Endoplasmic Reticulum While…

Being Synthesized

Soluble Proteins Made on the ER Are…

Released into the ER Lumen

Start and Stop Signals Determine…

the Arrangement of a Transmembrane Protein in the Lipid Bilayer

Membrane-enclosed organelles import proteins by one of three mechanisms.

All of these processes require energy; The protein remains folded during transport in mechanisms 1 and 3 but usually must be unfolded during mechanism 2.

The outer nuclear membrane is continuous with the…

ER membrane

The nuclear pore complex forms a gate through which selected macromolecules and larger complexes…

enter or exit the nucleus

Prospective nuclear proteins are imported from the cytosol through…

nuclear pores

nuclear localization signal

recognized by nuclear import receptors, which interact with the cytosolic fibrils that extend from the rim of the pore.

Energy supplied by GTP hydrolysis drives…

nuclear transport

The small monomeric GTPase, Ran, exists in two conformations…

one carrying GTP, the other GDP.

Ran-GAP (GTPase-activating protein)

the accessory protein that triggers GTP hydrolysis; found exclusively in the cytosol, where it converts Ran-GTP to Ran-GDP

Ran-GEF (guanine nucleotide exchange factor)

the accessory protein that causes Ran-GDP to release its GDP and take up GTP; found exclusively in the nucleus

The localization of these accessory proteins guarantees that the concentration of Ran-GTP is higher in the…., thus driving the…

nucleus, nuclear import cycle in the desired direction

Mitochondrial precursor proteins are unfolded during…

import

A mitochondrion has an outer and inner membrane, both of which must be…

crossed for a mitochondrial precursor protein to enter the organelle.

Precursor Protein

an inactive protein or peptide that can be turned into an active form by post-translational modification

To initiate transport, the mitochondrial signal sequence on a mitochondrial precursor protein is…

recognized by a receptor in the outer mitochondrial membrane

This receptor is associated with a protein translocator, which…

transports the signal sequence across the outer mitochondrial membrane to the intermembrane space

The complex of receptor, precursor protein, and translocator then diffuses laterally in the outer membrane until the signal sequence is…

recognized by a second translocator in the inner membrane

Together, the two translocators transport the protein across both the outer and inner membranes…

unfolding the protein in the process

The signal sequence is finally…

cleaved off by a signal peptidase in the mitochondrial matrix

Proteins are imported into chloroplasts by a similar mechanism. The chaperone proteins that help pull the protein across the membranes and help it to refold are not shown. Some of the energy needed for this protein translocation comes from the hydrolysis of ATP, which…

allows the chaperones to function

The endoplasmic reticulum is the most…

extensive membrane network in eukaryotic cells.

A common pool of ribosomes is used to synthesize all the proteins encoded by…

the nuclear genome.

Ribosomes that are translating proteins with no ER signal sequence remain…

free in the cytosol

Ribosomes that are translating proteins containing an ER signal sequence (red) on the growing polypeptide chain will be…

directed to the ER membrane

Many ribosomes bind to each mRNA molecule, forming a polyribosome. At the end of each round of protein synthesis, the ribosomal subunits are…

released and rejoin the common pool in the cytosol

An ER signal sequence and an SRP…

direct a ribosome to the ER membrane

The SRP (brown) binds to both the exposed ER signal sequence and the ribosome, thereby slowing…

protein synthesis by the ribosome

 The SRP–ribosome complex then binds to an…

SRP receptor (dark blue) in the ER membrane

The SRP is released, and the ribosome passes from the SRP receptor to a…

protein translocator (light blue) in the ER membrane. Protein synthesis resumes, and the translocator starts to transfer the growing polypeptide across the lipid bilayer.

A single-pass transmembrane protein is retained in the…

lipid bilayer

A double-pass transmembrane protein has an…

internal ER signal sequence

Define post-translational import

moving a protein into an organelle after it’s fully made

Define co-translational import

moving a protein into an organelle while it’s being made

What 3 ways do membrane-enclosed organelles import proteins?

proteins can enter through pores, across membranes, or via vesicles

What are signal sequences and what are their functions?

short stretches of amino acids that direct a protein to its correct location in the cell

What part of the nuclear envelope provides the entry for molecules going in and out of the nucleus.

the nuclear pore complex allows molecules to go in and out of the nucleus

What must a protein have to enter the nucleus?

a protein must have a nuclear localization signal (NLS) to enter the nucleus

What is a precursor protein?

an inactive, initial form of a protein that will be modified or activated later

What is the role of ribosomes and where can they be in the cell?

Ribosomes make proteins and can be found in the cytoplasm or attached to the rough ER.