Traffic Control -
February 21st, 2014
We have seen how a cell makes proteins, but to be of any use and to perform their assigned functions proteins have to get from the ribosome to where they are needed. This process is called protein trafficking. Some proteins need to be in the membrane, some need to be exported out of the cell, some need to be in the nucleus, some in the mitochondria, some need to make energy at just the right time and place. Without a mechanism to regulate where the newly synthesized proteins go chaos would reign and the cell would die. It would be like having the girders that support a building dumped in the lobby, or the natural gas that heats your house pumped into the bathroom instead of into the furnace.
How is the traffic controlled?
Special signals that facilitate interaction with transporters of many different types are built into the protein. For example there is a nuclear localization signal that allows a protein to stick to a structure called a nuclear pore, which looks like a life saver candy embedded in the nuclear envelope (the surrounding membrane of the nucleus). The nuclear pore is made up of perhaps 30 different proteins called nucleoporins.
Camillo Golgi, the discoverer of the Golgi apparatus won a Nobel Prize in 1906.
Proteins that need to go into the nucleus have special sequences of about 7 amino acids that stick to a protein called importin (because it imports its cargo protein, also it is importint!) and moves the protein into the nucleus. These special signals work like zip codes and facilitate the movement of proteins to the correct location. In some cases the proteins move; in which case they are said to be translocated, in combination with a specialized transporter called a chaperone (importin is an example of one chaperone). This movement can occur as the protein is being made or afterwards.
The endoplasmic reticulum (blue) receives proteins as they are made from ribosomes (red). Vesicles pinch off from the endoplasmic reticulum with protein attached and follow microtubule tracks to the Golgi apparatus (salmon). After processing in the Golgi apparatus the proteins once again become cargo in vesicles that pinch off. When the protein is to be released from the cell those vesicles fuse with the outer membrane of the cell and release their cargo. On the right is a very high magnification of a Golgi apparatus.Download for free at http://cnx.org/contents/7d5ecac2-a4c4-4167-b952-c3a9bde54252@5.
We discussed the endoplasmic reticulum (ER) in the last post and described it as a structure with two membranes decorated with ribosomes. Proteins are released into the space between the membranes and eventually small spheres of membrane material pinch off of the ER. Proteins that are supposed to be released to the outside of the cell, such as hormones, ride the sphere to another nearby structure called the Golgi apparatus, named for its discoverer. The spheres with their protein cargo move along cable like tracks called microtubules riding on a specialized motor called kinesin. Just like the monorail at Disney World!
The microtubule track is the blue green rod at the bottom. The large green spheres are vesicles with protein cargo. The sphere is carried along the track by motors called kinesin, the red and blue leg like structures. In a process that requires energy the legs walk along the track carrying their vesicle cargo!
Image courtesy of BioMEDIA ASSOCIATES, LLC
There is a great deal of controversy over how the proteins move through the Golgi apparatus and two main mechanisms are currently in favor, one is called “Kiss and Run”! But the proteins that need to do make their way through this structure and are then thought to wind up in a new sphere, this time pinched off from the membranes of the Golgi. These are called exocytic vesicles (exocytic because they will dump their “cargo” outside of the cell). These exocytic vesicles fuse with the plasma membrane (the membrane surrounding the cell) by sticking to special proteins called SNAREs and the protein is released to the outside.
This transport highway is a two-way street and proteins made by other cells can get inside by sticking to the plasma membrane on structures called receptors. They are pinched off into spheres containing the proteins called endocytic vesicles (endocytic meaning “inside the cell”). Some of these can fuse with the Golgi apparatus and are further processed in many ways. The proteins can be directed to the part of the cell that needs them. In some cases they can be broken down into component parts in a kind of cellular recycling plant for proteins that ensures the right amount of a protein is around.
All of this action occurs in all of our cells thousands and thousands of times each second!
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Originally published at lcresearchcenter.tumblr.com.
