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Flashcards in Protein Trafficking Deck (28)

The synthesis of all proteins begins where?

cytoplasmic ribosomes


Where do nuclear-encoded proteins function?

^cytoplasmic mitochondrial and nuclear proteins --> cytoplasmic ribosomes

-Cell membrane
-Secreted from the cell
^Membrane, secretory and lysosomal proteins --> ER bound for completion thru N terminus signal


What determines site of protein localization?

Information contained on primary structure of the protein


Cytoplasmic protein signal

If cytoplasmic there is no signal


Mitochondrial protein signal

30-50 aa pre-sequence with amphipathic character (hydrophobic and positively charged aa)


Nuclear protein signal



Membrane/secretory protein signal

20-30 AA signal sequence with core of hydrophobic amino acids preceded by an N terminal Methionine sometimes followed by + charged amino acids


Lysosomal protein signal



Co-translational insertion in ER membrane

For secretory, membrane, lysosomal proteins

-as secretory proteins are being translated onto cytoplasmic ribosomes, translation is interrupted after the 1st 50-70 amino acids are added to growing chain

-SRP binds the signal peptide as it exits the ribosome

-ribosome-mRNA-SRP complex directed to the ER membrane where SRP binds to SRP receptor

-SRP and SRP receptor bind GTP leading to GTP hydrolysis and release of SRP from the complex leaving the mRNA-ribosome complex on the ER membrane

-ribosome associates with translocon

-protein synthesis resumes and the polypeptide chain is inserted through the channel co-translationally

-once polypeptide is inside ER membrane, signal peptidase cleaves it


How are integral membrane proteins inserted into the ER membrane?

In the exact conformation that they will assume in the plasma membrane using a combination of hydrophobic start and stop sequences which either start or stop insertion into ER membrane


What can happen to proteins inside the ER?

1) Folding by Hsp70

2) Glycosylation with N-linked glycans added on asparagine residues

3)Formation of disulfide bonds

4)Folding by peptidyl prolyl isomerase for proline containing proteins


What does the Golgi do?

Bunch of sorting
*O-linked glycosylation of Ser/Thr residues
*trimming of N-linked glycans
*phosphorylation of sugar residues and sulfation of both tyrosine residues and some sugar chains


Lysosomal Proteins

Lysosomal hydrolase enzymes are tagged with mannose-6-phosphate in the Golgi

1) GIcNAC phosphotransferase (in cis Golgi) and enzyme in trans golgi removes the GIcNAC residue masking the mannose-6-phosphate

2) A mannose-6-phosphate receptor in the trans Golgi segregates these proteins in vesicles which bud off and fuse with endosomes


Lysosomal Storage Disease

I-cell Disease

Results from a defect in the GIcNAC phosphotransferase and the subsequent inability to direct lysosomal hydrolases to endosomes

Accumulation of lysosomal hydrolases in plasma as a result


Origin of mitochondrial proteins

most proteins encodes in the nuclear genomes and not mitochondrial genomes --> proteins are translated on cytoplasmic ribosomes and inserted into the mitochondrial membrane where translation is completed


Pre-sequence in mitochondrial proteins

Longer than signal peptide and has amphipathic character- helix with hydrophobic AA on one side and charged on the other


Folding of mitochondrial proteins

Most proteins targeted to the matrix

Proteins are maintained in an unfolded state in the cytoplasm, assisted by the binding of chaperone proteins Hsp70

Mitochondrial targeting sequence interacts with outer membrane receptor TOM 20/22

Binds to TOM40, the import channel

Once inside the matrix, targeting sequence is cleaved and the protein associates with several mitochondrial chaperone proteins which lets it assume its native conformation


Importance of proton motive force

Proton motive force is required for entry of proteins into the mitochondria

Membrane potential across the inner mitochondrial membrane aids in the insertion of the + charged residues in presequence



Nuclear pore complex
Form connection between nucleus and cytosol



Nucleoproteins (type of NPC)
8 Nups form a ring-like structure through which molecules can pass thru

Also stabilize the complex and facilitate transport through the pore


transmembrane Nups

anchor NPC in the nuclear envelope


structural Nups

stabilize the nuclear envelope at nuclear pores and provide scaffolding for assembling other peripheral Nups


FG Nups

Phenlyalanine-Glycine Nups
Form diffusion barrier and bind transport receptors

FG repeats line the pore and also create basket into cytosol --> restrict diffusion of larger moelcules (>40kDa)



Nuclear localization signal
Proteins with KKKRG bind to cytoplasmic importins




sometimes small proteins require active transport


Import into the Nucleus process

NLS on molecules is recognized by cytosolic proteins importins

Molecule bound importin also bind the F Nups and the filaments that extend into the cytosol from the NPC

Importins repeatedly bind dissociate and bind to F repeats to facilitate movement thru pore

Once inside, importin releases and moves back into cytoplasm


Export out of Nucleus

Nuclear export signal on proteins to be moved out are bound to nuclear export receptors (exportins)


Nuclear Lamina

Thick layer of fibrous proteins (intermediate protein)

-stabilize nuclear structure
-anchoring nuclear pore complexes