The Mysterious Golgi

There is no cell part with a greater disparity between importance and understanding than the Golgi Apparatus.  In a fitting symbol of its complexity, biologists can’t even agree on what it should be called, with alternate names including Golgi Body, Golgi Complex or simply Golgi.  The general functions of the organelle are well known, modifying organic compounds and shipping them to their intended destination.  That said, the knowledge gaps in how it accomplishes these roles and what enzymes are involved remain surprisingly wide since Camillo Golgi identified the organelle in 1898.

Under the microscope, the Golgi appears as a stack of flattened membranes.  These membranes, called cisternae, pass molecules from one level to the next, chemically modifying them along the way.  The cisternae, which typically number in the single digits, make up the four functional regions of the Golgi : cis-Golgi network (closest to the center of the cell), cis-Golgi, medial-Golgi and trans-Golgi (closest to the outer membrane of the cell).  Each functional region has a different set of enzymes with varying roles in modifying organic compounds such as proteins and lipids.

A typical trip through the Golgi for a protein starts in the endoplasmic reticulum, where the protein is built by ribosomes and folded as it moves along.  The protein is secreted from the ER via vesicles, which proceed to and merge into the cis-Golgi network.  After this point, the protein moves outward, through the cis-Golgi and medial-Golgi, being modified and altered in each cisternae.  By the time it is packaged and released from the trans-Golgi, the protein has been transformed from a winding strand of amino acids into a functional protein.  Some of the changes the Golgi might make to a protein would include glycosylation, addition of sugar groups, and phosphorylation, addition of phosphate groups.  The Golgi may also add a signal sequence to help guide the protein to its eventual location. 

As if the Golgi’s role isn’t wide-reaching enough, it has further impact on cell structure.  Lysosomes, digestive structures found in some cells, are formed with help from the Golgi.  It is also known that they have a currently undetermined role in apoptosis, the programmed death of the cell. 

These unknowns join with question marks at various stages of the protein modification process to make the Golgi an organelle shrouded in mystery.  Recent research gives reason to believe that increased understanding is on the way.  In an experiment advanced by an undergraduate student at Rensselaer Polytechnic Institute, researchers have developed an artificial Golgi Apparatus capable of modifying heparan sulfate, a complex sugar related to the important medicine heparin. 

Built on a chip, the synthetic Golgi is comprised of a maize-like structure, with different molecules and enzymes in each compartment.  Magnets move the molecules along through the structure.  While the design can only simulate one function of a true Golgi, it does allow experimentation on the organelle’s possible mechanisms that wouldn’t have been possible before.  Specifically, the researchers are trying to find a more effective way to acquire heparin, an anticoagulant used to prevent blood clotting.  Currently heparin is harvested from slaughtered animals, introducing high risk of contamination.

Whether it is modifying lipids, glycoslyating an enzyme or playing a role in cell death, the Golgi has undeniable importance to the cell.  Fortunately new research has brought a spotlight to this essential cell part.  Just as innumerable proteins owe their functionality to its contributions, the Golgi is owed more detailed treatment than the “packaging and shipping” label that is so often the beginning and end of its mention in discussions of the cell.

For more information:

Background on Camillo Golgi from the Nobel Prize :

http://nobelprize.org/nobel_prizes/medicine/articles/golgi/index.html

Golgi information from Cytochemistry.net :

http://www.cytochemistry.net/Cell-biology/golgi.htm