Eukaryote Translation.pdf

7/27/2019 Eukaryote Translation.pdf

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Translation

Ribosomes synthesize a polypeptide according

to the genetic instructions in mRNA.



Translation in Eukaryotes occurs in 3 steps

1. Initiation• Ribosome binds to the mRNA and initiates at an AUG

(methionine) codon

2. Elongation• The polypeptide is lengthened one amino acid at a time

3. Termination• Synthesis of the polypeptide terminates and the ribosome

dissociates from the mRNA and the polypeptide



Initiation of Translation (Eukaryotes)

• The small ribosomal

subunit forms an

initiation complex with

the initiator tRNA.

• This complex binds the 5’cap, then scans 5’ to 3’ in

search of the first AUG

codon.



Initiation of Translation

(Eukaryotes)• Once the first AUG is

located then the large

ribosomal subunit to binds toform the functional ribosome

• An aminoacyl-tRNA then binds to the A site and a

peptide bond is formed

between methionine and theamino acid in the A site

• Initiation is now complete

and elongation can proceed



Elongation

• The nascent (growing) polypeptideis bound to the peptidyl-tRNA inthe P-site

• The incoming aminoacyl-tRNA binds to the A-site

• Peptidyl transferase catalyzes a peptide bond between the newamino acid and the nascent polypeptide and breaks the bond between the nascent chain and the peptidyl-tRNA

• The ribosome translocates by onecodon along the mRNA



Elongation

• The tRNA in the E site

exits the ribosome

• The A site receives the next

incoming aminoacyl-tRNA

• A peptide bound is formed and the cycle continues

until termination



Termination

• Releasing factors bind to

termination codons (UAG,

UAA, UGA) in the A-site.

• Releasing factors facilitate

hydrolysis of the nascent polypeptide from the

peptidyl-tRNA thus

freeing the polypeptide

from the ribosome.



Termination

• Release of the polypeptide

is followed by dissociationof the ribosomal subunits

from the mRNA



The 5’ cap and poly-A tail increase translation rates.

• Specialized proteins bind to the5’cap and 3’ poly-A tail of themRNA. These proteins then bind to each other and thus

bring the 5’ and 3’ ends of themRNA together, forming acircle. Ribosomes that terminatetranslation are physically closeto the 5’ cap where they bind and begin translation again. Inthis way the 5’ cap and poly-A

tail function to increasetranslation rates.

• Many ribosomes translate thesame mRNA simultaneously.This complex is a polyribosome(polysome)



Post Translational Modifications

• The process of geneexpression is not finished

when an mRNA has been

translated.

• Many post translational

modifications may be required

for a polypeptide to fold into

the shape required for

function.



Post Translational Modifications

• Modifications may include:

– Phosphorylation, the addition of a

phospahate group

– Methylation, the addition of a

methyl group

– Glycosylation, the addition of

sugar groups

– Disulfide bonds, the formation of

covalent bonds between 2 cysteine

amino acids.

– Proteolytic Cleavage, the cutting

of a sequence of amino acids from

the polypeptide

– Subunit binding to form a

multisubunit protein



Protein Folding begins during Translation

• A polypeptide begins

to fold as soon as it

leaves the ribosome.

• Some polypeptides

can fold into their

complete, matureconformation without

help.

• However, most polypeptides required

chaperones to help

them fold properly.



Heat Shock Proteins (HSP) acts as

Chaperones that aid protein folding

• Chaperones bind to hydrophobic regions of the

polypeptide and shield them from the aqueous environment

until the entire polypeptide is translated. Then thechaperones help the protein to fold into its proper shape,

with the hydrophobic R groups in the interior of the protein



Misfolded proteins are destroyed by

Proteosomes

• The proteosome is a barrel-

shaped, multisubunit

protease. Misfolded proteins

enter one end and come out

the other as small chains of amino acids (peptides) that

are ultimately recycled.

• Proteosomes are abundant,making up 1% of total

cellular protein.



Ubiquitin binds to misfolded

proteins, targeting them for

destruction by the proteosome

•Misfolded proteins and proteins with

oxidized or abnormal amino acids areseen as abnormal by the cell.

Specialized enzymes attach chains of

ubiquitin to these abnormal proteins.Ubiquitin has affinity for the

proteasome and thus brings the

abnormal protein to the proteasomefor destruction. Almost without

exception, proteins that enter the

proteosome are first bound to

ubiquitin.



Plasma membrane proteins and secreted proteins

are post-translationally modified in the Rough

Endoplasmic Reticulum (RER) and the Golgi•Proteins bound for the plasma

membrane must first enter the

RER where they begin to fold and undergo chemical modification.

•Proteins leave the RER in

vesicles, which then fuse with thegolgi, delivering their protein

cargo.

•Proteins are further modified inthe golgi before being sent in

vesicles to the plasma membrane

where they are either secreted or

integrated into the membrane.

A i l di t t l tid t



A signal sequence directs a nascent polypeptide to

the rough endoplasmic reticulum

• Only proteins that have an

N-terminal signal sequence

can enter the ER.

• A signal recognition

protein(SRP) binds to the

signal sequence as it firstexits the ribosome and

brings the ribosome to the

ER.• The polypeptide is threaded

through the ER membrane

and into the ER lumen



A signal sequence directs a nascent polypeptide

to the rough endoplasmic reticulum



A signal sequence directs a nascent polypeptide

to the rough endoplasmic reticulum





the rough endoplasmic reticulum

• Once the polypeptide is

completely threaded into

the ER, the signal

sequence is cleaved off .

• Now the polypeptide

undergoes post-translational modification

and begins to fold

Protein glycosylation occurs in the ER and



Protein glycosylation occurs in the ER and

the Golgi

• Most proteins that enter the ER

will be glycosylated, which

means that an oligosaccharide(branched sugar group) will be

covalently attached to the

protein.• The sugar group may ultimately

be important to the proteins

function or it may simply act as

an address label required to get

the protein to its next cellular

destination.

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Eukaryote Translation.pdf