Protein Synthesis and sorting Flashcards
What is Translation?
mRNA is decoded in a ribosome to produce a specific amino acid chain, or polypeptide
What are the five major components are involved in translation?
Ribosomes, tRNA molecules, Aminoacyl-tRNA syntheses, mRNA molecules and Protein factors
attach amino acids to their appropriate tRNA molecules
Aminoacyl-tRNA synthetases
encode the amino acid sequence information
mRNA molecules
Protein factors function as what?
facilitate some of the steps of translation
Made of two dissociable subunits; large subunit and small subunit
Each subunit self-assembles from rRNA and proteins
come together only when binding mRNA
The bacterial Ribosome
How many and what are the binding sites of a Ribosome?
3 tRNA (A aminoacytl site, P peptidl site, E Exit site) mRNA binding site
an adaptor that binds both a specific amino acid and the mRNA sequences that specify the amino acid
tRNA
Once amino acid is attached to tRNA it is called
amino acyl tRNA
Messenger RNA Brings what Information to the Ribosome
Polypeptide Coding
mRNA is exported to the cytoplasm via binding to mRNA-binding protein that contains amino acid sequence …
nuclear export signals (NES) for transport through the nuclear pores
Start codon where translation starts (N-terminus of polypeptide) and the stop codon is where
where translation ends (C-terminus of polypeptide (UAG, UAA, UGA)
Initiation of translation in bacteria: Assembly of the 70S translation initiation complex occurs in ___ steps
3
Step 1: initiation of translation
Three initiation factors (IF1, IF2 and IF3) and GTP bind to the small ribosomal subunit
Step 2: initiation of translation
The initiator aminoacyl tRNA and mRNA are attached and bind to the small ribosomal unit
initiator tRNA is tRNA that carries an N formylmethionine→ tRNAfMet
Step 3: initiation of translation
The large subunit joins the complex resulting 70S initiation complex and GTP is hydrolysed
The start codon in eukaryotes and archaea specifies
methionine rather than N-formylmethionine
After binding to mRNA, the small ribosomal subunit begins translation at the
first AUG triplet it encounters
a common start sequence is ACCAUGG called a
Kozak sequence, the entire thing is a start codon
Chain elongation involves what three steps
Aminoacyl tRNA binding
Peptide bond formation and
Translocation
Termination: A stop codon goes to the A site and recognized by
Release factors
releases the polypeptide
Hydrolysis
Post translational processing
Must be chemically modified before they perform their normal functions
In bacteria which two groups are removed during Post translational processing
N-formyl group and the Met are removed
In eukaryotes which group is removed during Post translational processing
Met at the N-terminis
Some enzymes synthesize as
inactive precursors, and must be activated by removal of specific sequences at one end or the other
Internal stretches of amino acids are
removed to make an active protien
Chemical modifications of individual amino acid groups by what three ways
methylation, phosphorylation or acetylation
Ploypeptides may undergoto to prosthetic groups
glycosylation or binding
- inteins are removed from a one or more polypeptides
- Resulting segments create a continuous polypeptide chain
- can be intramolecular or intermolecular
Protein Splicing
What are the two types of protein targeting and sorting?
Cotranslational and Posttranslational import
Posttranslational
after translation is completed, the polypeptides which are destined to cell organelles. Requires presence of special targeting signals
movement of the polypeptide across or into the ER membrane is directly coupled to the translational process
Cotranslational import
facilitate protein folding and assembly
Molecular chaperones
Folding is accompanied with formation of disulfide bonds between
Cys in the pp chain
Improperly folded proteins can activate
several types of quality control mechanisms
Unfolded protein response (UPR)
detect misfolded proteins and enhance pathways for protein folding
recognizes misfolded proteins and exports or ‘retranslocates’ them back to cytosol for degrdation
ER-associated degradation (ERAD)
Proteins Released into the ER Lumen Are Routed to the
Golgi Complex, Secretory Vesicles, Lysosomes, or Back to the ER
Most proteins synthesized on rough ER are
glycoproteins
The initial glycosylation takes place in the __ as the polypeptide is being synthesized
ER
Default pathway:
Soluble proteins move from the Golgi complex to
Secretory vesicles
The other major group of polypeptides is destined to become
integral membrane proteins
The completed polypeptide chain remains embedded in the ER membrane and then
can be targeted to its proper destination
Posttranslational import into organelles that are
not part of the endomembrane system
Proteins imported into organelles after completion of translation are synthesized on
free ribosomes and released into the cytosol
