valgus nteesi initsiatsioon eukar ootides n.
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VALGUSÜNTEESI INITSIATSIOON EUKARÜOOTIDES. INITSIATSIOON. 40S subunit mRNA Initsiator tRNA Met-tRNA i Met 10 eukaryotic initsiation factors PolyA-binding protein 60S subunit. 40S subunit. 18S rRNA No anti-Shine-Dalgarno sequence DOES NOT RECOGNIZE mRNA START SITE. mRNA.

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initsiatsioon
INITSIATSIOON
  • 40S subunit
  • mRNA
  • Initsiator tRNA Met-tRNAiMet
  • 10 eukaryotic initsiation factors
  • PolyA-binding protein
  • 60S subunit
40s subunit
40S subunit
  • 18S rRNA
  • No anti-Shine-Dalgarno sequence
  • DOES NOT RECOGNIZE mRNA START SITE
slide4
mRNA
  • Monocystronic
  • CAP structure at the 5’ terminus
  • AUG start codon (Kozak sequence)
  • PolyA sequence at the 3’ terminus

A/GNNAUG

ORF

CAP

AAUAAA

AAAAAAAAA

5’

3’

mrna cap
mRNA CAP
  • Promotes recognition of the translation initiation site by the 40S subunit
  • Recognized by eIF4E

eIF4E

CAP

mrna cap1
mRNA CAP
  • Capping is linked to the early stages of transcription initiation and elongation.
  • A phosphohydrolase removes the gamma phosphate from the 5' end of the transcribed pre-mRNA.
  • Guanylyl transferase catalyzes the condensation of GTP with the 5' end of the pre-mRNA. This creates the unusual 5'-5' triphosphate linkage. Pyrophosphate is released.
  • The terminal guanosine nucleotide is methylated by guanine-7-methyl transferase. S-adenosyl-methionine is required as a cofactor for this reaction.
mrna kozak sequence
mRNA Kozak sequence
  • A/GNNAUGG
  • Promotes selection of right AUG start codon
  • Situated 40-100nt away from the CAP
mrna polya sequence
mRNA PolyA sequence
  • 150-200nt
  • Binds polyA-binding protein (PABP) that promotes mRNA 5’ – 3’ ends interaction that stimulate initiation of translation

IF4G

PABP

IF4E

CAP

polyA

initsiator trna met trna i met
Initsiator tRNA Met-tRNAiMet
  • Met-tRNAiMet
  • Not formylated
  • A1 : U72 base pair is important for right initiator tRNA selection
  • Three conserved G:C

base pairs in the

anticodon stem

slide10

eIF4A+eIF4G

+eIF4E=eIF4F

initiation factors
INITIATION FACTORS

eIF4F

  • attaches 40S to mRNA

eIF4G

eIF4A

eIF4E

  • 80S formation
  • IF release

eIF5B

  • adapter
  • binds PABP
  • binds 43S
  • binds CAP
  • Helicase

bind 40S

eIF3

eIF5

eIF1

eIF1A

eIF2

  • holds subunits
  • apart
  • attaches 40S to mRNA
  • Met-tRNAi
  • scanning
  • AUG selection
  • 40S recruitment
  • adapter
  • binds eIF3, eIF1 and eIF2
  • induces GTP hydrolysis
slide19

IRES (Internal Ribosome Entry Site)

  • 40S subunits bind mRNA in a CAP independent manner
  • Does not involve scanning
ires internal ribosome entry site
IRES (Internal Ribosome Entry Site)
  • Initiation factor dependent internal initiation
  • eIF3, eIF2, Met-tRNAi dependent internal initiation
  • Initiation factor and Met-tRNAi independent internal initiation
ires initiation factor dependent internal initiation
IRESInitiation Factor Dependent Internal Initiation
  • Encephalomyocarditis virus; Poliovirus; FGF2; eIF4G
ires initiation factor and met trnai independent entry site
IRES (Initiation factor and Met-tRNAi independentEntry Site)
  • Dicistroviruses:

Cricet paralysis virus,

Taura shrimp virus

  • 80S ribosomes can

initiate translation

translation initiation controll eif2 phosphorylation
TRANSLATION INITIATION CONTROLL: eIF2 phosphorylation
  • Phosphorylation converts eIF2 from a substrate to a competitive inhibitor of eIF2B
  • eIF2 is always present in excess of eIF2B
translation initiation controll eif2 phosphorylation2
TRANSLATION INITIATION CONTROLL: eIF2 phosphorylation
  • GCN4 – transcriptional activator of amino acid biosynthetic genes
  • GCN2 production is increased under amino acid starvation conditions
  • which activate GCN2
eif4e inhibitory proteins 4e bp
eIF4E inhibitory proteins: 4E-BP
  • Compete with eIF4G for a common binding site on eIF4E
  • Binding is regulated by phosphorylation :
  • hypophosphorylation – prevents eIF4G binding to eIF4E (growth factors, mitogens)
  • hyperphosphorylation – 4E-BP does not bind eIF4E (nutrient deprivation, stress)
eif4e inhibitory proteins maskin
eIF4E inhibitory proteins: MASKIN
  • 3’ end of most mRNA terminate with 150-200 nt. polyA tail
  • In frog oocytes arrested at the end of meiotic prophase polyA tail is only 20-40 nt
  • When the oocytes are stimulated to re-enter the meiotic divisions the polyA tails are elongated to about 150 bases
eif4e inhibitory proteins maskin1
eIF4E inhibitory proteins: MASKIN
  • Cytoplasmic polyadenylation
  • is controlled by CPE (cytoplasmic
  • polyadenylation element)
  • CPEB binds CPE
  • CPEB binds MASKIN
  • MASKIN binds eIF4E and prevents
  • eIF4G from binding eIF4G
  • When oocytes are induced to
  • complete meiosis, Aurora A catalyses
  • CPEB phosphorylation
  • CPEB stimulates polyA tail growth
  • Newly elongated polyA binds PABP
  • PABP interacts with eIF4E and
  • displaces MASKIN
  • eIF4G binds