protien targetting- btech biotechnology- cell biology

Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion. This delivery process is carried out based on information contained in the protein itself. Correct sorting is crucial for the cell; errors can lead to diseases.
Targeting signals are the pieces of information that enable the cellular transport machinery to correctly position a protein inside or outside the cell. This information is contained in the polypeptide chain or in the folded protein. The continuous stretch of amino acid residues in the chain that enables targeting are called signal peptides or targeting peptides. There are two types of targeting peptides, the presequences and the internal targeting peptides
Protein targeting is necessary for proteins that are destined to work outside the cytoplasm.
Protein targeting is more complex in eukaryotes because of the presence of many intracellular compartments.
The signals involved are also called sorting signals. They are regions on the targeted protein with certain amino acid sequences.

These signals interact with specific receptors, either on the target organelle or a carrier protein.
There are two basic forms of targeting pathways:

* post-translational targeting:
o nucleus
o mitochondria
o chloroplasts
o peroxisomes

* co-translational targeting (secretory pathway):
o ER
o Golgi
o lysosomes
o plasma membrane
o secreted proteins
Nuclear targeting:

* Unusual since 2-way traffic:
o in: proteins, DNA
+ DNA & RNA polymerases
+ transcriptions factors
+ histones etc.
o out: mRNA, tRNA, rRNA

* Proteins are not transported through the nuclear membrane but rather through a complex pore called the nuclear pore:
o comprised of about 100 different proteins
o proteins smaller than 20 kDa move by diffusion
o proteins larger than 20 kDa move by selective transport (nuclear localization signal)
+ cluster of 4-8 positively charged amino acids (example: PKKKRLV)
+ signal sequence binds to receptor on the pore called importin


Mitochondrial targeting:

* not well understood
* usually by post-translational targeting




Lysosomal targeting:

* Lysosomes are organelles that store enzymes which rapidly degrade other proteins and nucleic acids.
* A famous target sequence is "KDEL"
* Initial targeting via secretory pathway
* Final targeting occurs in the Golgi