Unraveling How Proteins Are Transported & Sorted in Cells

[EIRE2003]

Hi!
If I could get an answer to this question I could understand biology a lot easier. In the cell interior, the endoplasmic Reticulum manufactures proteins and are then transported by a vesicle to the Golgi Apparatus where they are modified or processed and sorted.

My question is, ''how do the molecules know where to go all the time, do they each carry a charge and so are attracted to opposite charges or what?''

And when the proteins are sorted in the Golgi they are tagged, how are they tagged?

 

[iansmith]

Pathways Through the Endoplasmic Reticulum (ER)
The decision to enter the ER is dictated by the presence of a signal sequence on the growing polypeptide.

The Signal Sequence

The signal sequence consists of the first portion of the elongating polypeptide chain (so the signal sequence occurs at the amino terminal of the polypeptide). Typical signal sequences contain 15 - 30 amino acids. The precise amino acid sequence varies surprisingly from one protein to the next, but all signal sequences include many hydrophobic amino acids.

If a signal sequence is present,

* translation ceases after it has been synthesized
* the signal sequence is recognized by and is bound by a signal recognition particle (SRP)
* the complex of ribosome with its nascent polypeptide and the SRP binds to a receptor on the surface (facing the cytosol) of the ER.
* the SRP leaves and translation recommences
* the growing polypeptide chain is extruded through a pore in the ER membrane and into the lumen of the ER.
* the signal sequence is usually clipped off the polypeptide unless the polypeptide is to be retained as an integral membrane protein.
* other proteins, called molecular chaperones, present in the lumen of the ER, bind the growing polypeptide chain and assist it to fold into its correct tertiary structure.
* sugar residues may be added to the protein. The process is called glycosylation and often is essential for proper folding of the final product, a glycoprotein.

Destinations of proteins synthesized within the ER

The membrane of the transport vesicle fuses with the membrane of the Golgi apparatus, merging their contents. Further steps of glycosylation may occur within the Golgi apparatus. The exact pattern of glycosylation determines the final destination of the proteins. There are two options.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/ProteinKinesis.html