Proteins are translated and modified with in the rough endoplasmic reticulum. In order to complete their modifications, they are transported in membrane-bounded vesicles to the cis side of the Golgi apparatus

PROTEIN TARGETTING

Septelia Inawati Wanandi

Transport vesicles from the rough ER fuse together and become the cis cisterna of the Golgi.

Once they reach the Golgi apparatus, additional modifications are made to the transported proteins by resident Golgi enzymes. These modifications are key in ensuring the proteins reach their final destinations once they leave the Golgi apparatus.

The Golgi apparatus is divided into distinct regions. These include the cis cisterna, nearest the ER, the centrally located medial cisternae, the trans cisterna, and the trans Golgi Network (TGN).

According to the cis maturation model, the proteins are transported through the Golgi stack as the cisterna containing them migrate, or mature, in a cis-to-trans direction. New vesicles from the ER continually supply new cis cisterna as trans Golgi network vesicles mature.

Within the trans Golgi network, proteins are sorted by their final destinations. This is accomplished by receptor molecules embeddedin the membrane of the TGN.

When proteins have reached their correct location within the TGN, the membrane at those locations buds off into vesicles. More than one protein can be contained within each transport vesicle.

Once released, the vesicles carry their cargo proteins to a final location. Possible destinations include the lysosome, the digestive organelle of the cell, and the plasma membrane, where the proteins can be released elsewhere in the organism.

Summary   

• 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 via the nuclear pore using a nuclear localization signal. - ER targeting (secretory pathway) via N-terminal signal sequences using SRPs with subsequent attachment to the ER, - 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 a ubiqutin-dependent process by specific proteases in a large cytosolic complex called the proteasome.