Download Lect20.ProteinSynthesis

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
Transfer RNA Structure and Functional Domains
Chemistry of tRNA Charging with Amino Acid
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.
Ribosome Composition
Prokaryotes
16S
30S
+ 21 proteins
rRNA
Subunit
23S
rRNA
70S
Ribosome
+
5S
+ 34 proteins
rRNA
50S
Subunit
Eukaryotes
18S
40S
+ ~25 proteins
rRNA
Subunit
28S
rRNA
80S
Ribosome
+
60S
Subunit
5S
rRNA
+ ~40 proteins
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.
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.
Dormant Eukaryotic mRNAs and Their Activation
Elongation Sequentially Adds Amino Acids to a Peptidyl-Acyl tRNA
Peptide(n)-acyl-tRNA + Aminoacyl-tRNA ---> tRNA(uncharged) + Peptide(n + 1)-acyl-tRNA
Elongation Factors and GTP Needed for tRNA Recruitment and Translocation
EF-Tu
GDP + Pi
GTP
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.
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)
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
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.
Type II and III TM Proteins Have Internal Hydrophobic Domains That
Cotranslationally Interact with ER Translocon and Resolve In Different Ways