Week 4
Chapter 15 intracellular compartments and protein transport
The cell: a complex, organized unit
In Eukaryotic cells, different metabolic processes are carried out in
different compartments
Why compartmentalize?
o Specialization in function
o Higher efficiency
o Portion off processes that are harmful for the rest of the cell
3 ways of protein import by organelles
1. Transport in and out the
nucleus through nuclear pores
Nuclear proteins are
synthesized by free ribosomes
o Nuclear proteins are fully
synthesized in the cytosol
o After protein folding, the
nuclear proteins are
imported from the cytosol through nuclear pores
Transport through nuclear pores
o Disordered protein segments fill the center of the channel
allowing small water-soluble molecules to pass in and out freely
and nonselectively
o Prospective nuclear proteins contain a nuclear localization signal
(NLS) consisting of several positively charged amino acids
o Binding of the NLS to a nuclear import receptor guides passage of
nuclear proteins by disrupting the pre meshwork
o Proteins are transported into the nucleus in their fully folded
conformation
o Signal sequence: amino acid sequence that directs a protein to a
specific location in the cell (generally 15-60 amino acids in length
The positively charged amino acids will locate on one side of
the helix
The apolar to a other side this combination is recognized by
receptors in the membrane of a mitochondrion
2. Transport across membranes
Mitochondria and chloroplasts
o Surrounded by two
membranes
o DNA (circular) and ribosomes:
within inner membrane
, o Large percentage of genes transferred to nucleus of host
o Most proteins are encoded by genes in the nucleus and imported
from the cytosol
Protein import used by mitochondria and chloroplasts
o Most mitochondrial and chloroplasts proteins are imported form
the cytosol
o These proteins are synthesized by free ribosomes in the cytosol
o Proteins typically unfold during this type of transport
o A specific signal sequence (at the N-terminal end) is recognized..
o By an import receptor coupled to a protein translocator
o Proteins unfold during transport
o Proteins are translocated across both membranes simultaneously
o The signal peptide is cleaved off after translocation
The ER is the most extensive
membrane in eukarytoic cell
functions:
o Attachment site for
ribosomes
o Protein folding and
modification
The ER: first stop of the
endomembrane system
o Proteins that function in organelles o the ondomembrane system
(ER, Golgi, endosomes, lysosomes and also peroxisomes), or
outside the cell, are formed by ribosomes that are attached to the
rough ER
Proteins enter the ER while being synthesized
o Two pools of ribosomes
Free ribosomes
Membrane-bound ribosomes, bound to the cytosolic side of
the ER lumen
o Ribosomes are directed to the ER membrane by an ER signal
sequence, which binds to a signal recognition particle (SRP) as it
emerges from the ribosome
Er signal sequence targets proteins to the ER
o SRP and SRP receptor act as molecular matchmakers that guide
ribosomes to the ER
Signal peptidase frees N-terminus of translocator
Proteins are released as soluble proteins in the lumen of the ER and
can be transported to other locations in transport vesicles
3. Transport by vesicels
This type of transport occurs between organelles of the
endomembrane system
The endomembrane system (ER, Golgi, endosomes, lysosomes and
also peroximes) mainly uses vesicles
Chapter 15 intracellular compartments and protein transport
The cell: a complex, organized unit
In Eukaryotic cells, different metabolic processes are carried out in
different compartments
Why compartmentalize?
o Specialization in function
o Higher efficiency
o Portion off processes that are harmful for the rest of the cell
3 ways of protein import by organelles
1. Transport in and out the
nucleus through nuclear pores
Nuclear proteins are
synthesized by free ribosomes
o Nuclear proteins are fully
synthesized in the cytosol
o After protein folding, the
nuclear proteins are
imported from the cytosol through nuclear pores
Transport through nuclear pores
o Disordered protein segments fill the center of the channel
allowing small water-soluble molecules to pass in and out freely
and nonselectively
o Prospective nuclear proteins contain a nuclear localization signal
(NLS) consisting of several positively charged amino acids
o Binding of the NLS to a nuclear import receptor guides passage of
nuclear proteins by disrupting the pre meshwork
o Proteins are transported into the nucleus in their fully folded
conformation
o Signal sequence: amino acid sequence that directs a protein to a
specific location in the cell (generally 15-60 amino acids in length
The positively charged amino acids will locate on one side of
the helix
The apolar to a other side this combination is recognized by
receptors in the membrane of a mitochondrion
2. Transport across membranes
Mitochondria and chloroplasts
o Surrounded by two
membranes
o DNA (circular) and ribosomes:
within inner membrane
, o Large percentage of genes transferred to nucleus of host
o Most proteins are encoded by genes in the nucleus and imported
from the cytosol
Protein import used by mitochondria and chloroplasts
o Most mitochondrial and chloroplasts proteins are imported form
the cytosol
o These proteins are synthesized by free ribosomes in the cytosol
o Proteins typically unfold during this type of transport
o A specific signal sequence (at the N-terminal end) is recognized..
o By an import receptor coupled to a protein translocator
o Proteins unfold during transport
o Proteins are translocated across both membranes simultaneously
o The signal peptide is cleaved off after translocation
The ER is the most extensive
membrane in eukarytoic cell
functions:
o Attachment site for
ribosomes
o Protein folding and
modification
The ER: first stop of the
endomembrane system
o Proteins that function in organelles o the ondomembrane system
(ER, Golgi, endosomes, lysosomes and also peroxisomes), or
outside the cell, are formed by ribosomes that are attached to the
rough ER
Proteins enter the ER while being synthesized
o Two pools of ribosomes
Free ribosomes
Membrane-bound ribosomes, bound to the cytosolic side of
the ER lumen
o Ribosomes are directed to the ER membrane by an ER signal
sequence, which binds to a signal recognition particle (SRP) as it
emerges from the ribosome
Er signal sequence targets proteins to the ER
o SRP and SRP receptor act as molecular matchmakers that guide
ribosomes to the ER
Signal peptidase frees N-terminus of translocator
Proteins are released as soluble proteins in the lumen of the ER and
can be transported to other locations in transport vesicles
3. Transport by vesicels
This type of transport occurs between organelles of the
endomembrane system
The endomembrane system (ER, Golgi, endosomes, lysosomes and
also peroximes) mainly uses vesicles
