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LECT 20: PROTEIN SYNTHESIS AND TRANSLATIONAL CONTROL. High fidelity of protein synthesis from mRNA is essential. Mechanisms controling translation accuracy include very high fidelity of (1) tRNA charging with correct amino acid

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slide1

LECT 20: PROTEIN SYNTHESIS AND TRANSLATIONAL CONTROL

High fidelity of protein synthesis from mRNA is essential. Mechanisms

controling translation accuracy include very high fidelity of

(1) tRNA charging with correct amino acid

(2) codon-directed delivery of correct charged tRNA to ribosome

Mechanisms for translation elongation and termination are very similar

in prokaryotes and eukaryotes, but mechanisms of initiation are

fundamentally different

Translation of eukaryotic mRNAs can be regulated at the step of

initiation by multiple mechanisms

Secreted and membrane-spanning proteins are co-translationally

delivered across membranes by “translocons” guided by signal

and anchor sequences in the nascent polypeptide chain

slide4

tRNA Synthetase Achieves Fidelity Through Substrate Specificity and Editing

A tRNA synthetase recognizes a specific tRNA by interaction with unique

structural features of that tRNA,

including the anticodon

loop sequence.

The tRNA synthetase’s

active site also binds

correct amino acid,

but sometimes also

a related amino acid.

Charging with the wrong

related amino acid is

uncharged by a

separate editing

domain.

slide5

16S

rRNA

+ 21 proteins

30S

Subunit

23S

rRNA

70S

Ribosome

5S

rRNA

+

+ 34 proteins

50S

Subunit

18S

rRNA

+ ~25 proteins

40S

Subunit

28S

rRNA

80S

Ribosome

5S

rRNA

+

+ ~40 proteins

60S

Subunit

Ribosome Composition

Prokaryotes

Eukaryotes

slide6

Translation Initiation Complex in Prokaryotes

Initiation factors deliver initiator fMet-tRNA to

mRNA initiation codon positioned at the “P” site

of the 30S ribosomal subunit.

Initiation codon is AUG preceded by “Shine-Delgarno”

sequence that is recognized by the 16S rRNA in

the 30S ribosomal subunit.

slide7

Translation Initiation Complex in Eukaryotes

Initiation factors deliver 40S ribosomal subunit and

Met-tRNA to 5’ methylated cap on mRNA. The

40S subunit then “scans” down mRNA and docks

at first AUG codon.

Initiation factor interaction with 5’ cap is facilitated

by prior recruitment of initiation factor to the mRNA

via polyA binding protein (PABP) bound to the

mRNA’s 3’ polyA tail.

slide9

Elongation Sequentially Adds Amino Acids to a Peptidyl-Acyl tRNA

Peptide(n)-acyl-tRNA + Aminoacyl-tRNA ---> tRNA(uncharged) + Peptide(n + 1)-acyl-tRNA

slide11

Release Factors Dock Recognize Stop Codons at A Site To Promote Termination

Upon release factor docking to stop codon, water is used instead

of amino group of charged tRNA as nucleophile to attack acyl bond

of peptidyl-acyl-tRNA. Hydrolysis releases peptide from tRNA and

ribosome.

slide12

Secreted Proteins Have N-Terminal Signal Sequence

Signal sequence is 10-15 residue hydrophobic stretch near N-terminus.

Signal sequence triggers secretion mechanism, is usually cleaved at

nearby downstream small amino acid (Gly-X, Ser-X, Ala-X)

slide13

Nascent Signal Sequence Recruits Signal Recognition Particle and Translocon

SRP recruitment to signal sequence arrests translation

SRP docks to andopens a translocon on the endoplasmic reticular membrane

Translation resumes with cotranslational threading into ER lumen

Lumenal peptidase cleaves off signal sequence

Completed protein transported within vesicles and released by exocytosis

slide14

Transmembrane Proteins Are Cotranslationally Inserted Into Membrane

Type I TM protein has signal sequence and downstream hydrophobic helical

transmembrane sequence and polar stop transfer element.

Type II and III TM proteins do not have signal sequence.

slide15

Type II and III TM Proteins Have Internal Hydrophobic Domains That

Cotranslationally Interact with ER Translocon and Resolve In Different Ways