BIOC0005 - PROTEIN TRAFFICKING EXAM QUESTIONS
AND ANSWERS WITH COMPLETE SOLUTIONS VERIFIED
What are the two components of the cell?
cytoplasm + nucleus
What are the two components of the cytoplasm?
cytosol + organelles
How does a protein know where to be transported in a cell?
Via sorting signals.
This can be via signal sequences - exposed, single stretch of amino acids often at end
of polypeptide chains.
Or signal patches - amino acids contributing to the signal are separate until after the
protein folds.
What are examples of signal sequences?
Signal sequences can be cleaved or uncleaved after targeting.
What general proteins are required for protein translocation across membranes?
Receptor (soluble or membrane proteins) to recognise signal and a channel/pore/fold in
the membrane through which to guide the protein.
All nuclear-encoded proteins begin synthesis in the cytosol and are fully
translated before translocation. What signal is present on these proteins
,Nuclear localisation signals (NLS), which can either be a signal patch or sequence of
lysine and arginine residues - string of positively charged aa.
This can be anywhere on the protein since the protein is imported into the nucleus in the
folded state.
How do proteins enter the nucleus?
Via the nuclear pores, which are holes in the nuclear envelope. These nuclear pore
complexes are made up of proteins called nucleoporins.
Small proteins (<5000 Da) can freely diffuse through the nuclear pores.
5-60 kDa can diffuse, but much slower (and so need a method of active transport for
realistic time-frame)
> 60 kDa need active transport
What receptor and channel is used for the transportation of proteins into the
nucleus?
Nuclear import receptor - importin
This is a soluble cytosolic protein that is part of the karyopherin family
Channel is nuclear pore. Nucleoporins have FG repeats that serve as the binding sites
for import receptors.
What is the name of the receptor used for transportation of proteins out of the
nucleus? How does signalling occur with export?
, exportin (nuclear export receptor), same family.
This is a soluble nuclear protein; nucleoporins have binding sites for this.
The protein has a signal sequence - the export signal may have a string of leucine
residues.
Explain the Ran cycle.
95% of Ran in the nucleus
Ran-GEF responsible for exchange of Ran-GDP to Ran-GTP in the nucleus by
removing GDP and adding GTP.
Ran-GAP causes hydrolysis of Ran-GTP to Ran-GDP in the cytosol.
Explain the process of nuclear import.
Cargo binds to cytosolic importin via NLS.
Transport from cytosol to nucleus via nuclear pore.
Cargo displaced by Ran-GTP; cargo delivered to nucleus.
Importin-cargo transport from nucleus to cytosol.
Dephosphorylation of Ran-GDP; Ran-GDP dissociates from receptors.
Importin can rebind cargo.
Explain the process of nuclear export.
Cargo with nuclear export signal and Ran-GTP bind to nuclear exportin.
Transport to cytosol through pore.
Dephosphorylation of Ran-GDP; cargo and Ran-GDP dissociate.
AND ANSWERS WITH COMPLETE SOLUTIONS VERIFIED
What are the two components of the cell?
cytoplasm + nucleus
What are the two components of the cytoplasm?
cytosol + organelles
How does a protein know where to be transported in a cell?
Via sorting signals.
This can be via signal sequences - exposed, single stretch of amino acids often at end
of polypeptide chains.
Or signal patches - amino acids contributing to the signal are separate until after the
protein folds.
What are examples of signal sequences?
Signal sequences can be cleaved or uncleaved after targeting.
What general proteins are required for protein translocation across membranes?
Receptor (soluble or membrane proteins) to recognise signal and a channel/pore/fold in
the membrane through which to guide the protein.
All nuclear-encoded proteins begin synthesis in the cytosol and are fully
translated before translocation. What signal is present on these proteins
,Nuclear localisation signals (NLS), which can either be a signal patch or sequence of
lysine and arginine residues - string of positively charged aa.
This can be anywhere on the protein since the protein is imported into the nucleus in the
folded state.
How do proteins enter the nucleus?
Via the nuclear pores, which are holes in the nuclear envelope. These nuclear pore
complexes are made up of proteins called nucleoporins.
Small proteins (<5000 Da) can freely diffuse through the nuclear pores.
5-60 kDa can diffuse, but much slower (and so need a method of active transport for
realistic time-frame)
> 60 kDa need active transport
What receptor and channel is used for the transportation of proteins into the
nucleus?
Nuclear import receptor - importin
This is a soluble cytosolic protein that is part of the karyopherin family
Channel is nuclear pore. Nucleoporins have FG repeats that serve as the binding sites
for import receptors.
What is the name of the receptor used for transportation of proteins out of the
nucleus? How does signalling occur with export?
, exportin (nuclear export receptor), same family.
This is a soluble nuclear protein; nucleoporins have binding sites for this.
The protein has a signal sequence - the export signal may have a string of leucine
residues.
Explain the Ran cycle.
95% of Ran in the nucleus
Ran-GEF responsible for exchange of Ran-GDP to Ran-GTP in the nucleus by
removing GDP and adding GTP.
Ran-GAP causes hydrolysis of Ran-GTP to Ran-GDP in the cytosol.
Explain the process of nuclear import.
Cargo binds to cytosolic importin via NLS.
Transport from cytosol to nucleus via nuclear pore.
Cargo displaced by Ran-GTP; cargo delivered to nucleus.
Importin-cargo transport from nucleus to cytosol.
Dephosphorylation of Ran-GDP; Ran-GDP dissociates from receptors.
Importin can rebind cargo.
Explain the process of nuclear export.
Cargo with nuclear export signal and Ran-GTP bind to nuclear exportin.
Transport to cytosol through pore.
Dephosphorylation of Ran-GDP; cargo and Ran-GDP dissociate.
