Section Section Q – Protein synthesisQ – Protein synthesis

Q1 Aspects of protein synthesisQ1 Aspects of protein synthesis Codon-anticodon interaction, Wobble,

Ribosome binding site, Polysomes, Initiator tRNAQ2 Mechanism of protein synthesisQ2 Mechanism of protein synthesis Overview, Initiation, Elongation, TerminationQ3 Initiation in eukaryotesQ3 Initiation in eukaryotes Overview, Scanning, Initiation, Elongation, Termin

ationQ4 Translational control and post-trQ4 Translational control and post-tr

anslational eventsanslational events Translational control, Polyproteins, Protein targetin

g, Protein modification, Protein degration

ContentsContents

Q1 Aspects of protein synthesis — Q1 Aspects of protein synthesis — Codon-Codon-anticodon interactionanticodon interaction

• In the cleft of the ribosome, an anti-parallel formation of three base pairs occurs between the codon on the mRNA and the anticodon on the tRNA.

Some highly purified tRNA molecules were found to interact with more than one codon, and this ability is correlated with the presence of modified nucleosides in the 5’-anticodon position, particularly inosine (formed by post-transcriptional processing of adenosine by anticodon deaminase)

Q1 Aspects of protein synthesis — Q1 Aspects of protein synthesis — WobbleWobble

• To explain the redundancy of the genetic code. 18 aa are encoded by more than one triplet codons which usually differ at 5’-anticodin base

5'-anticodon base is able to undergo more movement than the other two bases and can thus form non-standard base pairs as long as the distances between the ribose units are close to normal.

All possible base pairings at the wobble position

No purine-purine or pyrimidine-pyrimidine base pairs are allowed as ribose distances would be incorrect (Neat!).

U is not found as 5’-anticodon base

Wobble pairing: non Waston-crick base paring

Q1 Aspects of protein synthesis — Q1 Aspects of protein synthesis —

Ribosome binding siteRibosome binding site

• Solely for prokaryotic translation

• A purine-rich sequence usually containing all or part of the sequence 5'-AGGAGGU-3'

• Upstream of the initiation codon in prokaryotic mRNA

• To position the ribosome for initiation of protein synthesis

Shine-Delgarno element

Q1 Aspects of protein synthesis — Q1 Aspects of protein synthesis —

PolysomesPolysomes• Each mRNA transcript is read simultaneously

by more than one ribosome.  • A second, third, fourth, etc. ribosome starts to re

ad the mRNA transcript before the first ribosome has completed the synthesis of one polypeptide chain.

• Multiple ribosomes on a single mRNA transcript are called polyribosomes or polysomes. 

• Multiple ribosomes can not be positioned closer than 80 nt.

Polysomes

• Electron micrographs of ribosomes actively engaged in protein synthesis revealed by "beads on a string" appearance. 

Q1 Aspects of protein synthesis — Q1 Aspects of protein synthesis — Initiator tRNAInitiator tRNA• Methionine is the first amino acids inco

rporated into a protein chain in both prokaryotes (modified to N-formylmethionine) and eukaryotes.

• Initiator tRNAs are special tRNAs recognizing the AUG (GUG) start codons in prokaryotes and eukaryotes.

• Initiator tRNAs differ from the one that inserts internal Met residues.

Initiator tRNA, fMet-tRNAfMet in E. coli

Lacking alkylated A endorses more flexibility in recognition in base pairing (both AUG and GUG).

Initiator tRNA formation in E. coli

1. Both initiator tRNA and noninitiator tRNAmet are charged with Met by the same methionyl-tRNA synthetase to give the methionyl-tRNA

2. Only the initiator methionyl-tRNA is modified by transformylase to give N-formylmethionyl-tRNAfmet.

Q2 Mechanism of protein synthesis — Q2 Mechanism of protein synthesis —

OverviewOverview

Protein synthesis falls into three stages .• Initiation – the assemble of a ribosome on

an mRNA.• Elongation – repeated cycle of amino acid

delivery, peptide bond formation and movement along the mRNA (translocation);

• Termination – the release of the polypeptide chain.

Q2 Mechanism of protein synthesis — Q2 Mechanism of protein synthesis —

InitiationInitiation

• In prokaryotes, initiation requires

• the large and small ribosome subunits,

• the mRNA

• the initiator tRNA

• three initiation factors .

Size comparisons show that the ribosome is large enough to bind tRNAs and mRNA.

IF1 and IF3 bind to a free 30S subunits.

IF2 complexed with GTP then bind to the small subunits, forming a complex at RBS.

The initiator tRNA can then bind to the complex at the P site paired with AUG codon.

The 50S subunits can now bind. GTP is then hydrolyzed and IFs are released to give the 70S initiation complex

30S initiation complex

The assembled ribosome has two tRNA-binding sites, which are called A- and P-site, for aminoacyl and peptidyl sites respectively.Only fMet-tRNAfMet can be used for initiation by 30S subunits; all other aminoacyl-tRNAs are used for elongation by 70S ribosomes.

Q2 Mechanism of protein synthesis — Q2 Mechanism of protein synthesis —

ElongationElongation

• With the formation of the 70S initiation complex, the elongation cycle can begin.

• Elongation involves the three factors, EF-Tu, EF-Ts, EF-G, as well as GTP, charged tRNA and the 70S initiation complex.

The three steps of elongation

1.Charged tRNA is delivered as a complex with EF-Tu and GTP .

2.Peptidyl tranferase (50S ribosomal subunit) makes a peptide bond by joining the two adjacent amino acid without the input of more energy.

3.Translocase (EF-G), with the energy from GTP, moves the ribosome one codon along the mRNA, ejecting the uncharged tRNA and transferred the ribosome peptide from the mRNA.

EF-Tu-Ts exchange cycle

Peptide bond formation takes place by reaction between the polypeptide of peptidyl-tRNA in the P site and the amino acid of aminoacyl-tRNA in the A site.

Q2 Mechanism of protein synthesis — Q2 Mechanism of protein synthesis —

TerminationTermination

Protein factors called release factors interact with stop codon and cause release of completed polypeptide chain.

RF1 and RF2 recognizes the stop codon with the help of RF3

The release factors make peptidyl transferase transfer the polypeptide to water, and thus the protein is released

Release factors and EF-G: remove the uncharged tRNA and release the mRNA,.

Q3 Initiation in eukaryotes — Q3 Initiation in eukaryotes — OverviewOverview

• Most of the differences in the mechanism of protein between prokaryotes and eukaryotes occur in the initiation stage, where a greater numbers of eIFs and a scanning process are involed in eukaryotes.

• The eukaryotic initiator tRNA does not become N-formylated.

prokaryotic eukaryotic function

Initiation factor

IF1 IF3

IF2

eIF3 eIF4c eIF6 eIF4B eIF4F

eIF2B eIF2

eIF5

Bind to ribosome submits

Bind to mRNA

Initiator tRNA delivery

Displacement of other factors

Elongation factor

EF-Tu

EF-Ts

EF-g

eEF1α

eEF1βγ

eEF2

Aminoacyl tRNA delivery

Recycling of EF-Tu or eEF1α

Translocation

Termination factors

RF1, RF2, RF3

eRF

Polypeptides Chain release

Q3 Initiation in eukaryotes — Q3 Initiation in eukaryotes — ScanningScanning

• The eukaryotic 40s ribosome submit complex bind to the 5’cap region of the mRNA and moves along it scanning for an AUG start codon.

Eukaryotic ribosomes migrate from the 5’end of mRNA to the ribosome binding site, which includes an AUG initiation codon.

Q3 Initiation in eukaryotes — Q3 Initiation in eukaryotes — InitiationInitiation

• In contrast to the events in prokaryotes, initiation involves the initiation tRNA binding to the 40S subuits before it can bind to the mRNA. Phosphorylation of eIf2, which delivers the initiation tRNA, is an important control point.

Initiator tRNA+eIF2+GTP

eIF3+4C+40S

Ternary complex

43S ribosome complex

43S preinitiation complex

+mRNA+eIF4F+eIF4B

ATP

ADP+Pi 48S preinitiation complex

+

Scanning

More factors involved

Scanning to find AUG

Q3 Initiation in eukaryotes — Q3 Initiation in eukaryotes — ElongationElongation

• The protein synthesis elongation cycle in prokaryotes and eukaryotes is quite similar.

• The factors EF-Tu EF-Ts EF-G have direct eukaryotic equivalents called eEF1α eEF1βγ eEF2

Q3 Initiation in eukaryotes — Q3 Initiation in eukaryotes — TerminationTermination

• Eukaryotes use only one release factors eRF, which requires GTP,recognize all three termination codons.

• Termination codon is one of three (UAG, UAA, UGA) that causes protein synthesis to terminate.

Q4 Translational control and post-Q4 Translational control and post-translational events — translational events —

Translational controlTranslational control• In prokaryotes, the level of translation of di

fferent cistrons can be affected by:

• (a) the binding of short antisense molecules,

• (b) the relative stability to nucleases of parts of the polycistronic mRNA ,

• (c) the binding of proteins that prevent ribosome access.

In eukaryotes, 1.protein binding can also mask the

mRNA and prevent translation, 2.repeats of the sequence 5'-AUUUA -

3' can make the mRNA unstable and less frequently translated.

Q4 Translational control and post-Q4 Translational control and post-translational events — translational events —

PolyproteinsPolyproteins

• A single translation product that is cleaved to generate two or more separate proteins is called a polyprotein. Many viruses produce polyprotein.

Q4 Translational control and post-Q4 Translational control and post-

translational events — translational events — Protein Protein targetingtargeting• The ultimate cellular location of proteins is

often determined by specific, relatively short amino acid sequence within the proteins themselves. These sequences can be responsible for proteins being secreted, imported into the nucleus or targeted to other organelles.

Prokaryotic protein targeting: secretion

Eukaryotic protein targeting

• Targeting in eukaryotes is necessarily more complex due to the multitude of internal compartments:

• There are two basic forms of targeting pathways

2.1.

The secretory pathwayin eukaryotes (co-translational targeting)

• The signal sequence of secreted proteins causes the translating ribosome to bind factors that make the ribosome dock with a membrane and transfer the protein through the membrane as it is synthesized. Usually the signal sequence is then cleaved off by signal peptidase.

Q4 Translational control and post-Q4 Translational control and post-

translational events — translational events — Protein Protein modificationmodification• Cleavage:

– To remove signal peptide

– To release mature fragments from polyproteins

– To remove internal peptide as well as trimming both N-and C-termini

Q4 Translational control and post-translational eventQ4 Translational control and post-translational events — s — Protein degrationProtein degration

• Different proteins have very different half-lives. Regulatory proteins tend to turn over rapidly and cells must be able to dispose of faulty and damaged proteins.

Faulty and damaged proteins are attached to ubiquitins (ubiquitinylation).

The ubiquitinylated protein is digested by a 26S protease complex (proteasome) in a reaction that requires ATP and releases intact ubiquitin for re-use.

Protein degradation: process

• In eukaryotes, it has been discovered that the N-terminal residue plays a critical role in inherent stability.– 8 N-terminal aa correlate with stability:

Ala Cys Gly Met Pro Ser Thr Val

– 8 N-terminal aa correlate with short t1/2:

Arg His Ile Leu Lys Phe Trp Tyr

– 4 N-terminal aa destabilizing following chemical modification: Asn Asp Gln Glu

Multiple choice Multiple choice questionsquestions

1. Which statement about the codon-anticodon interaction is false?A it is antiparallel and can include nonstandard base pairs. B inosine in the 5' -anticodon position can pair with A, C or U in the 3'-codon position C inosine in the 3’-anticodon position can pair with A, C or U in the 5’-codon position.D A is never found in the 5'-anticodon position as it is modified by anticodon deaminase.2 ． Which one of the following statements correctly describes initiation of protein

synthesis in E. coli?A the initiator tRNA binds to the Shine-Dalgarno sequence. B three initiation factors are involved and IF2 binds to GTP. C the intermediate containing IF1, IF2, IF3, initiator tRNA and mRNA is called the 30S ini

tiation complex. D binding of the 50S subunit releases IF1, IF2, GMP and PPi. E the initiation process is complete when the 70S initiation complex is formed which cont

ains the initiator tRNA in the A site of the ribosome and an empty P site.

3 ． Which statement about elongation of protein synthesis in prokaryotes is false?

A elongation can be divided into three steps: peptidyl-tRNA delivery peptide bond formation and translocation.

B the peptidyl transferase center of the large ribosomal subunit is responsible for peptide bond for mation.

C in the EF-Tu-Ts exchange cycle EF-Tu-GTP is regenerated by EF-Ts displacing GDP.

D EF-G is also known as translocase and uses GTP in its reaction. 4 ． E. coli release factor 1 (RF1) recognizes which codons? A UAA only. B UAG only. C UGA only.D UGA and UAA.E UAG and UAA. F UAG and UGA.

5 ． Which two of the following statements about initiation of eukaryotic protein synthesis are true?

A eukaryotes use a mRNA scanning method to locate the correct start codon.B there are at least nine eukaryotic initiation factors (eIFs). C eukaryotic initiation uses N-formylmethionine. D the 80S initiation complex completes the initiation process and contains the

initiator tRNA base paired to the start codon in the A site. E ATP is hydrolysed to AMP and PPi during the scanning process. F the initiator tRNA binds after the mRNA has bound to the small subunit. 6 ． Which of the following protein synthesis factors are not equivalent p

airs in prokaryotes and eukaryotes? A EF-G; eEF2. B EF-Tu; eEF1α. C RF1 and RF3; eRF. D EF-Ts; eEFαβ

7 ． Which statement about post-translational events is false?

A some mRNAs encode polyproteins.

B protein targeting involves signal sequences in the nascent polypeptides.

C signal peptidase removes one or two amino acids from the amino terminus of some proteins.

D proteins can be modified by acetylation phosphorylation and glycosylation.

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