Protein sorting and protein traffic
Importance:
Eukaryotic cells are highly compartmentalised. The different
compartments contain different proteins (and other
macromolecules).
A eukaryotic cell contains 10 billion (~1010) protein molecules.
Cells are highly organised.
Different organelles contain different sets of proteins.
If proteins were not sorted correctly, chemical chaos would result.
Protein synthesis/ degradation.
It is important for proteins to be sorted to the correct compartment.
For example, the machinery required for protein synthesis
(ribosomes etc.) function in the cytoplasm. Many enzymes that
break down macromolecules are sorted and trafficked to lysosomes.
The lysosomal membrane separates these degradative enzymes
from the cytoplasm. Note: the pH of lysosomes is lower than the
cytoplasm and most of the enzymes in lysosomes are optimised to
function at low pH: acid hydrolases.
Protein sorting starts in the cytosol:
Virtually all protein synthesis occurs in the cytoplasm. Ribosomes
are cytoplasmic.
The exception is in mitochondria and chloroplasts (plants) – they
have their own protein synthesis machineries that synthesise
mitochondrial/ chloroplast encoded proteins. Note: the majority of
,these proteins are synthesised in the cytoplasm and have to be
imported into these organelles.
Sorting signals:
Proteins contain ‘sorting signals’ that provide information regarding
where it should be sorted to. Analogy: a postcode.
Cellular machineries recognise these sorting signals and facilitate
the sorting.
Sorting signals are specific stretches of amino acids in proteins.
3 types of sorting signals in proteins: those that are responsible for
sorting to the nucleus, to mitochondria and to the ER.
The sorting signals for different organelles are different and are
recognised by different machineries.
Nuclear targeting:
Often proteins that are to be targeted to the nucleus contain one or
more stretches of positively charged amino acids (Lysine, Arginine).
These strentches of amino acids are situated on the surface of the
protein once it has folded.
The importance of these stretches of positively charged amino acids
for nuclear targeting can be illustrated experimentally. T-antigen or
T-antigen with a single amino acid (K-T) change have been
expressed in cells in culture. The wild type (non-mutant) protein is
efficiently sorted to the nucleus, the altered (mutated) protein is not
targeted to the nucleus. The simplest explanation is that disrupting
this stretch of positively charged amino acids means that the protein
no longer contains a nuclear import signal.
Nuclear pores:
, The nuclear membrane is a double membrane contiguous with the
ER.
All movement of molecules into and out of the nucleus occurs
through nuclear pores.
Each pore is composed of a large number of distinct protein
subunits. Fibrils protrude from both side of the complex.
Small molecules can diffuse freely into (and out of) the nucleus.
Large macromolecules (e.g. protein, nucleic acids) require active
transport through nuclear pores (i.e. require input of energy).
Importance:
Eukaryotic cells are highly compartmentalised. The different
compartments contain different proteins (and other
macromolecules).
A eukaryotic cell contains 10 billion (~1010) protein molecules.
Cells are highly organised.
Different organelles contain different sets of proteins.
If proteins were not sorted correctly, chemical chaos would result.
Protein synthesis/ degradation.
It is important for proteins to be sorted to the correct compartment.
For example, the machinery required for protein synthesis
(ribosomes etc.) function in the cytoplasm. Many enzymes that
break down macromolecules are sorted and trafficked to lysosomes.
The lysosomal membrane separates these degradative enzymes
from the cytoplasm. Note: the pH of lysosomes is lower than the
cytoplasm and most of the enzymes in lysosomes are optimised to
function at low pH: acid hydrolases.
Protein sorting starts in the cytosol:
Virtually all protein synthesis occurs in the cytoplasm. Ribosomes
are cytoplasmic.
The exception is in mitochondria and chloroplasts (plants) – they
have their own protein synthesis machineries that synthesise
mitochondrial/ chloroplast encoded proteins. Note: the majority of
,these proteins are synthesised in the cytoplasm and have to be
imported into these organelles.
Sorting signals:
Proteins contain ‘sorting signals’ that provide information regarding
where it should be sorted to. Analogy: a postcode.
Cellular machineries recognise these sorting signals and facilitate
the sorting.
Sorting signals are specific stretches of amino acids in proteins.
3 types of sorting signals in proteins: those that are responsible for
sorting to the nucleus, to mitochondria and to the ER.
The sorting signals for different organelles are different and are
recognised by different machineries.
Nuclear targeting:
Often proteins that are to be targeted to the nucleus contain one or
more stretches of positively charged amino acids (Lysine, Arginine).
These strentches of amino acids are situated on the surface of the
protein once it has folded.
The importance of these stretches of positively charged amino acids
for nuclear targeting can be illustrated experimentally. T-antigen or
T-antigen with a single amino acid (K-T) change have been
expressed in cells in culture. The wild type (non-mutant) protein is
efficiently sorted to the nucleus, the altered (mutated) protein is not
targeted to the nucleus. The simplest explanation is that disrupting
this stretch of positively charged amino acids means that the protein
no longer contains a nuclear import signal.
Nuclear pores:
, The nuclear membrane is a double membrane contiguous with the
ER.
All movement of molecules into and out of the nucleus occurs
through nuclear pores.
Each pore is composed of a large number of distinct protein
subunits. Fibrils protrude from both side of the complex.
Small molecules can diffuse freely into (and out of) the nucleus.
Large macromolecules (e.g. protein, nucleic acids) require active
transport through nuclear pores (i.e. require input of energy).
