Lecture 31 Post-Translational Protein Modifications

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Exam#2

Cytoplasmic ribosomes vs. Rough E.R. ribosomes

a)secrete proteins found in cytoplasm, nucleus, and mitochondria
b) Secreted protein in vessicle after processing through Rough E.R. and cis->trans golgi. (never contact cytoplasm)

If protein lacks an E.R. signal Sequence then

those proteins are made on free ribosomes and released to cytosol

Proteins with organelle specific signal sequences are

are first released to cytosol then through organ membrane to matrix/stromal space

The Three steps of the secretory pathway are

1) Protein Synth on R.E.R.-> move into RER lumen
2) Protein folding and modification by chaperones in lumen
3) Transport to golgi, lysosomes, or cell surface, via budding/vessicle fusion

What are the Specifics of secretory step 1/ moving to lumen?

translated on rough E.R. ribosomes that stud its surface. After protein synthesis begins on FREE RIBOSOMES a 16-30 AA residue E.R. signal sequence is translated.
AA=amino acid
This E.R. signal sequence has one or more positively charged AA's followed by a continuous stretch of hydrophobic residues known as the core & is on the N-terminus of the protein.

What does the Core do?

It directs the Free Ribosomes that is translating future secreted protein to attach to the rough E.R. and initiates translocation of the growing peptide.

What does the signal recognition particle do?

It binds the 16-30 AA residue E.R. signal sequence and the large subunit of the ribosome to STOP translation before ribosome is attached to the Rough E.R.

(this pauses translation so that it can resume again once on the RER because this protein will be secreted)

When does translation begin again?

After SRP-signal sequence-ribosome complex binds the SRP Receptor on the Rough ER then Translation may begin again. Followed by steps 2-3 of secretory path.

What does the SRP receptor do, specifically?

It orients the ribosome to bring it in contact with the PROTEIN TRANSLOCATOR (donut shaped protein inside RER)

The tunnel of the large ribosome where mRNA used to sit aligns with the hole of donut shaped Protein Translocator while the Signal recognition protein detaches.

What is the name of the pore that allows the Protein into the RER?

Translocon, which is part of the protein translocator. The elongating peptide chain (soon to be full protein) passes through this hole into the lumen of the RER.

Does the growing polypeptid ever encounter the cytoplasm?

No. Large 60S ribosomal subunit is aligned in such a way that it is not exposed to cytoplasm.

Is the growing polypeptide of secretory proteins capable of folding outside the RER?

No. Folding of secretory proteins only occurs in the RER lumen.

When is the 16-30 AA signal sequence cleaved? and by what?

When inside of the RER lumen it is cleaved by signal peptidase.

How do these fully translated proteins get to the golgi?

Transport vessicles take these proteins to the Golgi (think of golgi as a post office for sorting and packaging) things always move from the cis->trans end of the golgi.

All protein secreting cells use what pathway?

The SECRETORY pathway, which utilizes exocytosis to release vessicle contents to post marked organelle, organ, or extracellular destination.

Each integral protein has a unique orientation to the membranes phospholipid bilayer. Is this orientation preserved during transport?

Yes.

What are the 4 modifications to secreted and membrane bound proteins prior to reaching final destination?

addition of carbohydrates
formation of disulfide bonds
proper folding and assembly of multisubunit proteins
specific proteolytic cleavages

__________ builds oligosaccharide carbs on the side chain of _____,______, & _______

Glycosyl transferase, serine, threonine, asparagine

Where are disulfide bonds formed in eukaryotes?

ONLY in the lumen of the E.R., thus they are only found in secretory proteins

Sulfhydryl groups also known as _________ occur between __________ residues

Thiol, Cysteine

Uptake of solid particles known as _________ occurs through polymerization and depolymerization of what filament?

Phagocytosis, actin

After a solid particle binds the cells surface what is the name of the projections that surround the particle?

psuedopods

Phagocytized particles in vessicles later fuse with what?

Lysosomes

What is pinocytosis

Cell drinking-retrieving cytoplasm after exocytosis of vessicle

What does receptor mediated endocytosis require?

A particle bound to a receptor

What are the 3 uses of receptor mediated endocytosis?

Uptake of nutritive substances (LDL or Iron transferrin)
Waste disposal (getting rid of used up proteins)
Mucosal transfer (antibodies move through single layer of epithelium across mucosal layer)

What is an endosome?

Pinocytic vessicle + ENDOcytic vessicle= acidified (ph 5) endsome
Material can be transferred from endosome to golgi

Often endosome fuses with _______ to form _________.

lysosome, secondary lysosome

What function does mannose 6 phosphate serve?

It acts as a postage stamp residue to route protein enzymes of the lysosome like glycosidases, proteases, phosphatases, and sulfatases to the lysosome organelle

(note this this is an oligosaccharide)

Name two lysosomal storage disease.

Tay Sachs & Gaucher

Autophagy vs. Macroautophagy?

lysosomes degrade bad proteins vs. large double membrane enclosing an entire defective organelle for DESTRUCTION!!!!!

What is Crohn disease?

an aberrant immune response to components of the normal bacterial flora in the intestine. Occurs in 100 per 100,000

Which gene is most often associated with Chrohns?

single-nucleotide polymorphism (SNP) in the ATG16L1 (autophagy-related 16-like 1) gene: involved with AUTOPHAGY

What is a hypothesized consequence of this gene alteration?

Impaired sequestration allows certain bacteria to live longer, thus inducing an inflamatory immune response in the gut.

Chaperones are also known as _______ ________ proteins.

Heat shock

What three things happen to misfolded proteins?

misfolded proteins are recognized, transferred across ER membrane, degraded

What marks a misfolded protein for destruction?

Ubiquitin. First, a ubiquitin-conjugating enzyme (E1) activates ubiquitin and transfers it to the E2 component of a ubiquitin ligase (E2-E3 complex).
The E3 component of the ubiquitin ligase recognizes the target protein and transfers the ubiquitin from E2 to the target protein.

4 ubiquitins are required before transfer to proteasome for degradation

Know the proteosome structure

proteasome is a hollow cylinder whose inner surface is lined with proteases, covered with a large cap on both sides.

What do ubiquitin ligases recognize on misfolded proteins?

Some ubiquitin ligases recognize the presence of OXIDIZED AMINO ACIDS in the protein, others recognize ABNORMAL HYDROPHOBIC PATCHES on the surface of partially denatured proteins, and still others recognize SEQUENCE MOTIFS that are normally buried in the center of the protein but become exposed in misfolded proteins.

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