Protein Targeting Signals Flashcards
What is the nature of the signal targeting a protein to the ER?
Signal sequence recognised by SRP during translation
Where is the signal to target proteins to the ER located within gut primary sequence?
N-terminus
Are proteins targeted to the ER older or unfolded during transfer?
Unfolded - synthesised through membrane
Which specialist proteins are involved with targeting proteins to the ER?
SRP, SRP receptor, others
Is the signal to target proteins to the ER retained or cleaved
Cleaves by signal peptidase
Does targeting proteins to the ER require energy?
Yes - hydrolysis of GTP by SRP
What is the nature of the signal to target proteins to the nucleus?
NLS = nuclear localising signal
BASIC (Arg/Lys)
May be multipartite
Where is the signal to target proteins to the nucleus located?
Various positions, must be on the surface of the folded protein
Are proteins targeted to the nucleus folded or unfolded during transfer?
Folded - large pores in double membrane of nucleus
Which specialist proteins are involved in the targeting of proteins to the nucleus?
Importing recognises NLS and mediates transport; RanGTP displaces it in nucleus and drives out export cargo
Is the signal to target proteins to the nucleus retained or cleaved?
Retained (facilitates re-importing of proteins when nucleus reforms after cell division)
Does targeting proteins to the nucleus require energy?
Yes - hydrolysis of GTP
Describe the nature of the signal to target proteins to the mitochondria
Amphipathic signal for initial targeting to Matrix - may be extra signals to other final destinations
Where is the signal to target proteins to the mitochondria located within the primary sequence?
N terminus (or internal stop transfer = hydrophobic)
Are proteins targeted to the mitochondria folded or unfolded during transfer?
Held partially unfolded by chaperones e.g. MSF
What specialist proteins are involved in targeting proteins to the mitochondria?
Mitochondrial-import stimulating factor (MSF), tom and Tim channel complex
Is the signal to target proteins to the mitochondria cleaved or retained?
Cleaved
Does targeting proteins to the mitochondria require energy?
Yes - ATP hydrolysis by mHsp70 drives translocation; MSF uses ATP to keep some precursors unfolded
Describe the nature of the signal to target proteins to lysosomes
Post translational addition of Mannose-6-phosphate
Where is the signal to target proteins to the lysosomes located?
Must be signal patch to distinguish lysosomal proteins from other mannose-labelled proteins
Are proteins targeted to the lysosomes folded or unfolded during transfer?
Folded, delivered via vesicle
What specialist proteins are involved in targeting proteins to lysosomes?
Mannose-6-Phosphate receptor in trans-Golgi
Is the signal to target proteins to the lysosomes retained or cleaved?
Phosphate removed by phosphatase
Does targeting proteins to lysosomes require energy?
Yes - phosphotransferase ultimately requires ATP - but not direct
Describe the nature of the signal for protein retention in the ER
KDEL (Lys-Asp-Glu-Leu)
Where is the signal to retain proteins in the ER located on the primary sequence?
C-terminus
Are proteins for retention in the Er folded or unfolded during transfer?
Folded (delivered via vesicle)
What specialist proteins are involved in the retention of proteins in the ER?
KDEL receptor in cis Golgi
Is the signal to retain proteins in the ER cleaved or retained?
Retained
Does retaining proteins in the Er require energy?
No - involves binding and release dependent on pH
