05 - Protein Synthesis

chaperone proteins

bind to polypeptides during translation to prevent misfolding and aggregation

• ensure only properly folded proteins proceed

• quality control mechanism

polysome

multiple ribosomes can bind one mRNA transcript

• initiation is rate-limiting → main control point of translation

• cells regulate protein levels by controlling initiation factors

cytosolic vs ER translation

all translation starts in the cytosol

• proteins with signal sequence is recognized and bound by signal recognition protein (SRP), halting translation

• SRP translocates ribosome and docks at ER, resuming translation

• signal sequence cleaved by signal peptidase

• BiP chaperone protein binds to newly synthesized protein to prevent folding post-translationally until entire sequence has been translated

• proteins made in ER are put into vesicles to be delivered to Golgi for modification, and then are sent to plasma membrane, lysosome, or secreted

• proteins made in cytosol stay within cell

membrane proteins

orientation in membrane depends on signal sequences and stop-transfer signals

• N-terminus or C-terminus may face cytosol or lumen

• transmembrane proteins remain embedded during trafficking

post-translational modifications

• N-glycosylation: add sugar to aid in folding, recognition, and protection

• disulfide bridges: stabilize protein structure

• GPI anchors: tether proteins to outer plasma membrane

N-glycosylation

• oligosaccharide is transferred from a lipid carrier to polypeptide chains during translocation across the ER membrane

• important for recognition on extracellular surface

• protects against proteasomes

GPI-linkage

attachment of proteins to outer surface of plasma membrane without spanning lipid bilayer

• protein has hydrophobic signal sequence at C-terminus

• in ER, enzyme recognizes the sequence and cleaves off hydrophobic sequence

• protein is transferred onto a preassembled GPI anchor in ER membrane

• gycosyl sugar group connects protein to phosphatidylinositol that embeds into membrane bilayer

• protein is trafficked through Golgi to plasma membrane for attachment to outer leaflet

• protects cells from destruction by neutrophils

disulfide bonds

protein disulfide isomerase (PDI) catalyzes formation and breaking of disulfide bonds until proper shape or linkage is achieved

• quality control for proper folding

C-terminal KDEL sequence

retention signal for proteins that belong in the ER lumen

• proteins with tag remain in ER

• if escaped, KDEL receptor binds to it and packages protein into vesicles for retrograde transport to ER

• ensures ER helper proteins stay where they are needed