24 Questions
What is the primary function of the Pex14p receptor on the peroxisome membrane?
To recognize and bind the PTS1R complex
What is the role of chaperons in protein targeting to mitochondria?
To bind to the protein in the cytoplasm and ensure it reaches the correct organelle
What is the purpose of ATP hydrolysis in protein targeting to peroxisomes?
To allow the recycling of the PTS1R receptor
What is the function of signal peptidase in mitochondrial protein targeting?
To remove the signal sequence from the protein
What is the role of the PTS1R complex in peroxisome targeting?
To recognize and bind to the Pex14p receptor on the peroxisome membrane
What is the purpose of the protein translocator complex in mitochondrial protein targeting?
To transport the protein across the mitochondrial membrane
What is the purpose of chaperons in the mitochondrial protein targeting pathway?
To prevent premature folding of the protein
What is the primary function of the signal recognition particle (SRP) during protein targeting?
Recognizing the signal peptide and the ribosome during protein targeting to the ER
What is the destination of the PTS1R complex in peroxisome targeting?
The peroxisome membrane
During co-translational insertion of membrane proteins, what sequence allows hydrophobic amino acids to associate with the lipid membrane?
Anchor sequence
What is the primary site of protein synthesis for secretory proteins and membrane proteins?
Endoplasmic reticulum (ER)
What is the role of the signal sequence in protein targeting?
Binding to the SRP to facilitate protein targeting to the ER
Which of the following is NOT a characteristic of signal sequences?
C-terminal sequence
What is the primary mechanism of protein targeting to the ER?
Co-translational insertion
What is the primary cause of osteogenesis imperfecta, a family of genetic disorders that affect the bones?
Mutations in COL1A1 and COL1A2 genes that prevent the production of type I collagen
What is the role of signal sequences in collagen synthesis?
Directing polypeptide chains into the ER lumen
What is the fate of the procollagen molecules after they are secreted into the extracellular matrix?
They are cleaved by collagen peptidases to form tropocollagen
What is the characteristic of collagen fibrils in Ehlers-Danlos syndrome?
Increased extensibility and decreased rigidity
What is the term for the breaking of peptide bonds to remove part of a protein?
Proteolytic cleavage
What is the destination of proteins synthesized by ribosomes on the rough ER?
Plasma membrane
What is the peroxisome targeting sequence (PTS) typically located on?
C-terminus of the protein
What is the receptor that recognizes the peroxisome targeting sequence (PTS)?
PTS1R
What is the requirement for protein sorting that is intrinsic to the protein?
A signal
What is the term for the synthesis of polypeptides on ribosomes at the border of the ER?
Co-translational insertion
Study Notes
Protein Targeting to Peroxisomes
- Peroxisome targeting sequence (PTS) is a signal intrinsic to the protein, usually present on the C-terminus
- Cytosolic receptor protein PTS1R binds to the SKL signal and directs it to the correct membrane
- 14 Pex proteins create a transport channel through the peroxisomal membrane, facilitating the transport of the PTS1R complex across the membrane
- ATP hydrolysis is needed to allow recycling of the PTS1R receptor
Protein Targeting to Mitochondria
- Proteins contain a signal sequence to ensure they go to the right organelle
- Chaperons become associated with protein in cytoplasm (requires ATP), unfolding proteins to fit through the membrane
- Protein attaches to the “Protein Translocator Complex” and moves into mitochondria
- Chaperons are released, and another class binds to prevent premature folding
- Signal peptidase removes signal sequence, and protein is folded into final shape for its function
Collagen Biosynthesis
- Occurs in fibroblast cells
- Signal sequences direct polypeptide chain into the ER
- Hydroxylation of Proline and lysine residues (requires ascorbate + vitamin C)
- Glycosylation of selected hydroxylysine residues
- Assembly of 3 polypeptide chains into a procollagen triple helix
- Procollagen is processed into secretory vesicles, and secreted into extracellular matrix
- Procollagen’s terminal ends are cleaved by Collagen Peptidases, forming tropocollagen
- Tropocollagen molecules are covalently linked via aldol covalent cross-links, forming collagen fibrils by Lysyl Oxidase
Scurvy and Ehlers-Danlos Syndrome
- Scurvy is caused by insufficient hydroxylation of Proline and lysine residues due to lack of vitamin C and ascorbic acid
- Results in fatigue, weakness, poor wound healing, anaemia, and gum disease
- Connective tissue becomes defective, leading to instability of collagen
- Treatment involves vitamin C intake
- Ehlers-Danlos syndrome (EDS) is caused by mutations in genes affecting extracellular peptide cleavage and altering collagen fibril cross-linking
- Results in altered stability and functionality of fibers, making them more extensible and losing their rigidity
Protein Synthesis and Sorting
- Proteins synthesised on free ribosomes are destined for cytosol or post-translational import into organelles
- Proteins synthesised by ribosomes on the rough ER are destined for the plasma membrane or secretory pathway via co-translational insertion
- Co-translational insertion is the synthesis of polypeptides on ribosomes at the border of the ER, resulting in the polypeptide being contained within the ER
- The 4 requirements for Protein Sorting are: a signal intrinsic to the protein, a receptor that recognizes the signal, a translocation machinery, and energy
This quiz covers the role of PTS1R in protein targeting, its interaction with the Pex14p receptor, and the translocation machinery involved in transporting the PTS1R complex across the peroxisomal membrane.
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