Protein Trafficking

Organelles have   – part of what creates the identity of each organelle and how reactions
are compartmentalized

 nvolves methods in which proteins reach their target organelles

Test Question Protein translated in cytosol are transported to different sites than those translated on the surface
of ER – location of  is important

3 mechanisms of protein sorting
1.  
2.  
3.  

Proteins made in the cytosol enter the nucleus through
the   (function:  )

Proteins can be transported across the organelle
membrane by  (primarily for chloroplasts,
mitochondria, peroxisomes and ER)

Proteins translated into the ER are redistributed to other
organelles by  

Proteins contain  (a stretch of specific amino acid sequences)

Proteins lacking signal sequences are  

The signal sequences can be “swapped” through genetic engineering to make a protein end up in a different cell
location ( 

TEST QUESTION Nuclear envelope has pores that are highly regulated –  

Test Question This is the only way in which molecules can enter or exit the
 

Macromolecules require   to cross the  

Protein import requires  ‐‐‐Pro‐Lys‐Lys‐Lys‐Arg‐Lys‐‐ (most well known)

Test Question:   (similar to  ) bind to “cargo” protein

Nucleus and cytoplasm are connected by a channel filled
with flexible, mobile filamentous proteins termed  

 ‐cargo enter from the cytoplasm and hop
between the binding sites on   until they reach the
nuclear side of the   (unknown mechanism)

Cargo released from karyopherin by action of  

 divided into five families: Ras, Rho, Rab,
Arf, Ran

When  = GTPases are active (ON) and interact
with GAP to promote signaling by P (phosphate) transfer
(GTP hydrolysis)

When   = GTPases are inactive (OFF) and no
signals generated – bound to GDP

 removes GDP and replaces with GTP on GTPases

  stimulates GTP hydrolysis by GTPases

Test Question
RanGAP
• Localized  of NPC
• Stimulates  
• Ran in   =  ‐ 

Test Question
RanGEF
• Localized in 
• Exchanges   for  
• Ran in   =  ‐ 

Nuclear import of proteins (To get IN)
• Importins (karyopherins) binds to   of protein cargo
• Importins attach to  
• Importin‐protein move through 

• At the nuclear interface –  binds to importin
• It destabilizes importin‐protein interaction
• Protein released

•  ‐importin can transport back through the NPC.   stimulates   of  to   +
 

Nuclear export of proteins
• Protein cargo must have a  
• Exportin binds to the   of cargo protein

•   stabilizes exportin‐cargo protein interaction
• Exportin functions similar to importins through the NPC
• At the cytoplasmic interface – RanGAP stimulates hydrolysis
of  
• Exportin and cargo protein are released
• A transporter for exportins returns the molecules to the
nucleus

• ****RanGDP adds to the   in the cytoplasm or is
transported by   back into the nucleus where  
converts it into  (dissociates  )

Test Question Entry into mitochondria requires   transactors because they have an outer and inner membrane

Transport into mitochondria is   and requires a cleavable  ‐terminal signal sequence

TEST QUESTION
Occurs through protein translocators:
•   ‐> translocase of the outer membrane
•  ‐> translocase of the inner membrane

 goes to   and the proteins can be unfolded with  

  – end up either being secreted or on the
inside of an organelle
  – end up in all different
membranes

TEST QUESTION Proteins generally enter the while being synthesized
called 

ER signal sequence is directed to the ER membrane with the help of:
•   – a cytosolic protein that binds to the ER signal sequence
•   receptor – a receptor bound to the ER membrane that binds 

TEST QUESTION Binding of the   to the signal sequence directs the new protein and ribosome complex to attach to the   at the   receptor

signal sequence causes the protein translocator to  (protein is still being made)

A signal peptidase cleaves the   sequence of the protein

Once the  of the protein passes through
the protein translocator it is not free

  capture the protein and fold it into the
proper conformation as it is essential to function

Membrane protein  have start and stop sequences for insertion into ER membrane

TEST QUESTION (Co-translational import) A   has a single
transmembrane region with the  in the ER
lumen and the  in the cytosol

WHen there are mulitople transmembrane regions, internal  is used to initiate translocation – but it is not cleaved off (hydrophobic
sequences form an  )

WHen there are mulitople transmembrane regions, Translocation continues until a  signal is reached

TEST QUESTION Once translocation initiates it 
so the protein must enter the organelles correctly –
importance of proper start‐transfer signals and
signal sequences

TEST QUESTION   that are retained in the lumen of the ER
contain the sequence Lys‐Asp‐Glu‐Leu ( ) on their  ‐
terminus –called an  

If the proteins are not retained, the proteins (and membrane lipids) are
transported via vesicles to other parts of the
 

The endomembrane system include  , ,   and  
• All make up the since their functions are coordinated
• Each surrounded and defined by a  

TEST QUESTION   transport is a major cellular activity, responsible for molecular traffic between a variety of specific  compartments.

Vesicular transport is a  process

  is responsible for protein synthesis, folding and quality
control

  is responsible for lipid and cholesterol synthesis,
steroid hormone secretion, metabolism of carbohydrates and
store/ release of calcium ions.

  ‐> responsible for protein packaging and
secretion

 ‐> responsible for the breakdown of different
biomolecules, contains enzymes for molecule breakdown
(nuclease, protease, lipase) and is liable for intra and extra
cellular absorption.

 ‐> responsible for the transportation of molecules
across the membrane.

TEST QUESTION Vesicles that bud from membranes are coated with specific  

TEST QUESTION The best studied vesicles are coated with  

TEST QUESTION: CCVs bud from the   on their way to the cell membrane or from the plasma membrane 

TEST QUESTION   binds to adaptin to initiate budding

  bind and cause the neck of the bud to constrict and pinch off (  hydrolysis required for the
process)

TEST QUESTION Newly formed vesicles are   (  and   are removed) and free to be carried to a target organelle by motor proteins

After uncoating – motor proteins transport vesicles along   or  to an organelle
destination

TEST QUESTION
 on the vesicle and organelle membrane form a complex that drives membrane fusion thereby delivering
the proteins to the organelle (  mediated vesicle fusion)

1. Secreted proteins enter  as they are being
synthesized by ribosome.
2. Protein exits   in vesicle.
3. Protein travels through the cisternae of the  
4. Protein enters a   that fuses with the
cell membrane.
5. Protein is secreted from the cell.