15.Posttranslational modifications and protein transport.
Covers how post-translational modifications diversify proteins and help them cross membranes, then follows where ribosomes place newly made proteins, including the signal peptide and Signal Recognition Particle route into the ER. It also describes the nuclear localisation sequence, mitochondrial and chloroplast targeting, and the fates of ER-synthesised proteins. The note closes with a brief list of other modifications: ubiquitination, nitrosylation, methylation and lipidation.
Topic 15 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 4 sections, about 388 words (3 min of reading), with 20 flashcards.
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Posttranslational modifications and protein transport.
Role of Post-Translational Modifications (PTMs)
Ribosomes & Protein Destination
Additional Post-Translational Modifications
Role of Post-Translational Modifications (PTMs)
PTMs create huge protein diversity and allow identical proteins to perform different functions in different cell types.
PTMs at the N-terminal or C-terminal help proteins cross membranes (ER, mitochondria, chloroplasts, lysosomes, plasma membrane).
Secretory proteins (hormones, enzymes, toxins) undergo PTMs and are synthesized in the endoplasmic reticulum (ER).
Proteolytic cleavage of amino-terminal signal sequences occurs as proteins cross membranes.
Proteins can be folded by chaperonins after translation.
PTMs can involve addition of sugars, lipids, phosphate groups, altering the protein’s function and activity.
Ribosomes & Protein Destination
All ribosomes begin translation in the cytosol.
Free ribosomes → make proteins staying in the cytoplasm.
Bound ribosomes (on ER) → make proteins for the endomembrane system.
Endomembrane System Includes:
Nuclear envelope
ER
Golgi apparatus
Lysosomes
Signal Peptide Mechanism
Proteins destined for the ER contain a signal peptide (~20 amino acids) near the N-terminus.
Signal Recognition Particle (SRP) binds the signal peptide and escorts the ribosome to the ER membrane.
SRP binds its receptor on the ER surface.
The growing polypeptide is threaded into the ER through a protein pore.
Once inside the ER, the signal peptide is removed.
Proteins with no signal sequence remain free in the cytosol.
Protein Folding & Targeting in Cytosol
Cytosolic chaperones help proteins fold correctly.
Final destinations of cytosolic ribosome products:
Cytosol
Nucleus
Mitochondria
Nuclear Localization Sequence (NLS)
7–41 amino acids
Rich in lysine & arginine
Directs proteins through nuclear pores into the nucleus.
Fates of Proteins Synthesized in the ER
Signal sequence retention → protein stays inside the ER.
Glycosylation → proteins sent to lysosomes or incorporated into plasma membrane.
Transport to Golgi → further modification → secretion outside the cell.
Protein Targeting to Mitochondria
Mitochondrial-destined proteins have a mitochondrial signal sequence.
Recognized by MSF (mitochondrial stimulation factor), a chaperone.
MSF delivers the protein to receptors on the outer mitochondrial membrane.
Additional factors translocate proteins to the matrix or inner membrane.
Protein Targeting to Chloroplasts
Chloroplasts have their own genome but rely heavily on nuclear-encoded proteins.
Proteins contain a transit sequence recognized by chloroplast receptors.
Cytosolic chaperones help direct the protein to:
Stroma
Thylakoid membrane
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