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From CellBiology

COP I

Vesicle transport is the main mechanism by which proteins are moved to and from the cell, as well as within the cell. COP I is a protein which coats vesicles being transported from the cis face of the golgi apparatus to the rough endoplasmic reticulum, which is referred to as endocytosis. It also has some role in transporting proteins from the rough endoplasmic reticulum to the golgi apparatus (exocytosis). COP I is an acronym for 'coat protein complex' acting on the endocytic transport of proteins. Cop I initiates the budding process of the cis network.

Protein transport between compartments within cell is achieved through coat vesicles. This transport is facilitated by one of three different coat vesicles:

  • 1. Clatherin: transport from plasma membrane of cell to trans golgi network (endocytosis);
  • 2. COPII: responsible for transport from ER to golgi apparatus (exocytosis); and
  • 3. COPI: responsible primarily for transport from cis golgi to RER (endocytosis), also to aids in exocytosis, and transports within the golgi apparatus. (Spang. A., et al 1998)

Structure

AS mentioned above, COPI is a coating protein, similar to COPII. It is consists of a coatomer protein, and ADP Ribosylation Factor 1. A COPI contains seven subunits which makes the heptamerical coat complex. It is considered to be one of the best characterised coat complexes.

Formation

The assembly of coat protein COPI is regulated by ADP ribosylation factor 1 (ARF1) and Sar1 GTPase. (Aridor. M. et al 1995). The formation of COPI is a process which has been debated in the past, but the consensus is that the following is the most correct process. The target membrane from which the COPI is initiated recruits ARF, and converts it to the active form of ARF-GTP. Coatomer, which is on the surface of coated proteins and vesicles which form and capture proteins for transport, attracts ARF to attach to the membrane of these vesicles. The activated ARF then forms an anchor by which coatomer is attached. (Spang. A)

Budding

The process of budding of the transport vesicles is initiated when the ARF protein swaps its bound GDP for GTP, this process is catalysed by enzymes within the golgi apparatus structure. The ARF-GTP compound is then attracted to and bound to ARF receptors on the golgi cisternae (cis face). Coatomers follow and bind to the cis face of the golgi cisternae also, and polymerise to form a fibrous coat. This coat, in turn, induces vesicle budding. Fatty acyl is necessary for the final separation of the vesicle from the initial membrane, but there is little knowledge on how this works. The newly budded vesicle is then ready for transport. This transport is directed by VSNARES, which are bound onto the vesicle surface during the budding process. VSNARES direct the vesicle towards the appropriate acceptor membranes of target cell. Once the vesicle has found the appropriate target cell, hydrolysis of the GTP bound to the ARF causes depolymerisation of the coat, in turn releasing the coatomers and the ARF-GTP. (Tainano, 2005)

Function

The function of COP I is to transport and direct vesicles from the golgi apparatus to the endoplasmic reticulum in eukaryote cells. Therefore, are involved in endocytosis of materials to the cell nucleus. It also has a lesser role in exocytosis of proteins, from the Endoplasmic reticulum to the golgi apparatus, however this is primarily done by COP II. COP I performs the essential task of collecting proteins, and breaking through the bilipid layer of the membrane of the target cellular compartment (Duden. R. 2003)


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References

Spang. A., Matsuoka. K., Hamamoto. S., Schekman. R., Orci. L. (1998). Coatomer, Arf1p, and nucleotide are required to bud coat protein complex I-coated vesicles from large synthetic liposomes. PNAS 95(19) 11199-11204.

Aridor. M., Bannykh. S., Rowe. T., Balch. W. (1995). Sequential coupling between COPII and COPI vesicle coats in endoplasmic reticulum to Golgi transport. The Journal of Cell Biology 131. 875-893.

Tainano. (2005). Tainano Nanobiotechnology Glossary. In Index of /chin. Retrieved May 10, 2009, from http://tainano.com/chin/Molecular%20Biology%20Glossary.htm.

Duden. R. (2003). ER-to-Golgi transport: COP I and COP II function (Review).Molecular Membrane Biology. 20(3).197-207.

LECTURE FEEDBACK

Lecture 4- What I found interesting about the nucleus and did not know, is that is had a double nuclear envelope. I also found it interesting that the outer envelope is continuous with the endoplasmic reticulum

Lecture 5- I find it difficult to understand how the proteins being transported out of the cell know where to go, and how to get there?!

Lecture 7- Apoptosis is a cellular process which requires lots of energy from mitochondria (produces ATP ie energy).

Lecture 8- N-CAM: Neural cell adhesion molecule. Ng-CAM: Neuron glia cell adhesion molecule. L-CAM: A neuronal cell adhesion molecule of the L1 protein family? I-CAM: Inter-cellular adhesion molecule

Lecture 10-

Lab 6- I found that the phenotype of the control B35 cells differed markedly from the Tm4 over-expressing B35 cells in that the control there were overall many more cells. There were nearly 4 times as many stumped cells in the wild type, and about 2 times as many prolonged and stringed. Also, there were no observed pygnotic cells in the Tm4 ever-expressing B35, however there were a few in the wild type. The over-expression of Tm4 may have led to the inhibition of cell formation, or of cells undergoing mitosis (division). This leads to the observed decrease in overall cell numbers.

Lecture 14-