Monday, 7 June y
BIOC2003
Intracellular protein trafficking
Overview
• Eukaryotic cells are divided into membrane-bound compartments called organelles
• each of these organelles have their own subset of proteins that need to reach the right
place for the cell to function properly
• the different organelles have specific functions, requiring particular proteins
• proteins encoded in the nucleus begin their synthesis in the cytosol
• from the cytosol they can be transported to:
• nucleus
• mitochondria
• ER
• plastids e.g. chloroplasts
• peroxisomes
Sorting signals
• a protein will know where to go in a cell because it has sorting signals
• the most common sorting signal is a signal sequence, which is an exposed, single
stretch of amino acids, often at the end of the polypeptide chain (N terminus or T ter-
minus)
• a signal patch is less common, it is when the amino acids contributing to the signal
are separate until after the protein folds
• the location of the proteins in the cell is crucial
• Examples of signal sequences:
• Nuclear import signal sequences have lysine and arginine-rich sequences (KKKRK)
• Mitochondrial import signal sequences are amphipathic alpha helices
• ER import signal sequences have hydrophobic amino acids
• Signal sequences can be cleaved or uncleaved after targeting (once the protein
reaches its target)
Transport across membranes
• the nucleus and the mitochondrion have double membranes
• the peroxisome gets rid of toxic substances via oxidation
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, Monday, 7 June y
• proteins are large macromolecules so in order for them to cross the membrane they
need a pore or a channel
• a small protein will easily fit through the pore but larger proteins require active trans-
port and thus make much more contact with the nuclear pore proteins
• there will always be:
• a receptor to recognise the signal - can be a soluble or a membrane bound receptor
• channel through which to guide the protein
• receptors and channels are made of proteins
Protein transport into the nucleus
• the nuclear localisation signals (NLS, lysine and arginine rich) can be anywhere in the
protein
• the nucleus has a double membrane
• nuclear pores make the channels in the membrane. Nuclear pores have 50-100 pro-
teins making up the complex. They are very large with many different components. It
is between 60-80 million daltons making it one of the largest protein assemblies.
• proteins (nucleoporins) in the nuclear pore complex form cage-like structures going
into the nucleus
• proteins enter the nucleus via the nuclear pores
• small molecules, less than 5kdaltons can freely diffuse through nuclear pores
• larger molecules are imported via active transport
• the nuclear basket is attached to a ring like structure in the nuclear membrane
• in nuclear protein import the protein is folded before it is imported and it has the NLS
attached to it
• the nuclear import receptor is called an importin. It is a soluble receptor that is part of
the karyopherin family. The receptor takes the folded protein to the nuclear pore
(channel).
• the importins when bound to the protein (recognises the lysine and arginine rich sig-
nals) take the protein to the nuclear pore
• nucleoporins in the nuclear channel have “FG repeats” that serve as binding sites for
the importin
• once the importin has bound the protein is taken into the nucleus
Nuclear Protein Export
• the protein is folded and has an export signal
• a string of leucine residues can act as an export signal
• the nuclear export receptor is called an exportin
2
BIOC2003
Intracellular protein trafficking
Overview
• Eukaryotic cells are divided into membrane-bound compartments called organelles
• each of these organelles have their own subset of proteins that need to reach the right
place for the cell to function properly
• the different organelles have specific functions, requiring particular proteins
• proteins encoded in the nucleus begin their synthesis in the cytosol
• from the cytosol they can be transported to:
• nucleus
• mitochondria
• ER
• plastids e.g. chloroplasts
• peroxisomes
Sorting signals
• a protein will know where to go in a cell because it has sorting signals
• the most common sorting signal is a signal sequence, which is an exposed, single
stretch of amino acids, often at the end of the polypeptide chain (N terminus or T ter-
minus)
• a signal patch is less common, it is when the amino acids contributing to the signal
are separate until after the protein folds
• the location of the proteins in the cell is crucial
• Examples of signal sequences:
• Nuclear import signal sequences have lysine and arginine-rich sequences (KKKRK)
• Mitochondrial import signal sequences are amphipathic alpha helices
• ER import signal sequences have hydrophobic amino acids
• Signal sequences can be cleaved or uncleaved after targeting (once the protein
reaches its target)
Transport across membranes
• the nucleus and the mitochondrion have double membranes
• the peroxisome gets rid of toxic substances via oxidation
1
, Monday, 7 June y
• proteins are large macromolecules so in order for them to cross the membrane they
need a pore or a channel
• a small protein will easily fit through the pore but larger proteins require active trans-
port and thus make much more contact with the nuclear pore proteins
• there will always be:
• a receptor to recognise the signal - can be a soluble or a membrane bound receptor
• channel through which to guide the protein
• receptors and channels are made of proteins
Protein transport into the nucleus
• the nuclear localisation signals (NLS, lysine and arginine rich) can be anywhere in the
protein
• the nucleus has a double membrane
• nuclear pores make the channels in the membrane. Nuclear pores have 50-100 pro-
teins making up the complex. They are very large with many different components. It
is between 60-80 million daltons making it one of the largest protein assemblies.
• proteins (nucleoporins) in the nuclear pore complex form cage-like structures going
into the nucleus
• proteins enter the nucleus via the nuclear pores
• small molecules, less than 5kdaltons can freely diffuse through nuclear pores
• larger molecules are imported via active transport
• the nuclear basket is attached to a ring like structure in the nuclear membrane
• in nuclear protein import the protein is folded before it is imported and it has the NLS
attached to it
• the nuclear import receptor is called an importin. It is a soluble receptor that is part of
the karyopherin family. The receptor takes the folded protein to the nuclear pore
(channel).
• the importins when bound to the protein (recognises the lysine and arginine rich sig-
nals) take the protein to the nuclear pore
• nucleoporins in the nuclear channel have “FG repeats” that serve as binding sites for
the importin
• once the importin has bound the protein is taken into the nucleus
Nuclear Protein Export
• the protein is folded and has an export signal
• a string of leucine residues can act as an export signal
• the nuclear export receptor is called an exportin
2
