Without Pics Assignment Protein Sorting and Targeting
Without Pics Assignment Protein Sorting and Targeting
Without Pics Assignment Protein Sorting and Targeting
PROTEIN TARGETING
A typical mammalian cell contains up to 10,000 different kinds of proteins; a
yeast cell, about 5000. For a cell to function properly, each of its numerous proteins must
be localized to the correct cellular membrane or aqueous compartment e.g., the
mitochondrial matrix, chloroplast stroma, Lysosomal lumen, or cytosol. Hormone
receptor proteins, for example, those that must be delivered to the plasma membrane if
the cell is to recognize hormones, and specific ion-channel and transporter proteins are
needed if the cell is to import or export the corresponding ions and small molecules.
Water-soluble enzymes such as RNA and DNA polymerases must be targeted to the
nucleus; still others, such as proteolytic enzymes or catalase, must go to Lysosomes or
peroxisomes, respectively. Many proteins, such as hormones and components of the extra
cellular matrix, must be directed to the cell surface and secreted.
Protein targeting is necessary for proteins that are destined to work outside the
cytoplasm.
1. Nucleus
2. Mitochondria
3. Chloroplasts
4. peroxisomes
1. Endoplasmic reticulum(ER)
2. Golgi Complex
3. Lysosomes
4. Plasma membrane
5. Secreted proteins
Nuclear targeting:
Nuclear targeting is unusually done by two-way traffic, i.e. transporting in
includes proteins, transcription factors histones etc. and out are mRNA, tRNA, rRNA.
Proteins are not transported through the nuclear membrane but rather through
complex pore called Nuclear pore, comprising of about 100 different proteins. Smaller
proteins move through pores by diffusion and larger move by selective transport, or
Nuclear Localization Signal. It is a cluster of positively charged amino acids, e.g.
PKKKRLV. Signal sequence binds to receptor on the pore called Importin.
Lysosomal targeting:
Lysosomes are organelles that store enzymes which rapidly degrade other proteins
and nucleic acids. A famous target sequence is "KDEL" Initial targeting is via secretary
pathway. Final targeting occurs in the Golgi complex.
Endoplasmic reticulum targeting:
It is co-translational insertion of protein into or through ER membrane via
ribosome (RER).
CONCLUSION
• Targeting of newly synthesized proteins is an integral component of protein synthesis.
• In prokaryotes, targeting is usually achieved by an N-terminal signal sequence of
about 20 mostly hydrophobic amino acids.
• In eukaryotes, targeting is more complex due to the large number of different cellular
compartments.
• Nuclear targeting is done via the nuclear pore using a nuclear localization signal.
• ER targeting is done via N-terminal signal sequences using SRPs with subsequent
attachment to the ER.
• Targeting is followed by transport to the Golgi complex.
• Protein degradation of damaged or obsolete proteins is carried out by Lysosomes,
vesicles filled with degradating enzymes in an ubiqutin-dependent process by specific
proteases in a large cytosolic complex called the proteasome.
REFERENCES:
Book: Molecular Cell Biology
Authors: Harvey Lodish Arnold Berk S. Lawrence Zipursky Paul
Matsudaira David Baltimore James E. Darnell
http://bass.bio.uci.edu/%7Ehudel/bs99a/lecture27/lecture8_2b.html