Question

describe what elements are critical for COP II and clathrin coat formation to nucleate.

Answer 1

The coat proteins forms a protenaceous coat around the protein to be delvered.They helps to form a vesicle which later fuses with the target membrane. Mainly 3 types of coated vesicles are there- COP II vesicles, COP I vesicles and Calthrin coated vesicles. All these are different with their coat elements and also in their target position.

COP II is for the transport of proteins from endoplasmic reticulum (ER) to golgi. COP I is to transport protein from golgi to endoplasmic reticulum. Calthrin coated vescicles transport proteins from plasma membrane and golgi to the endosomes.

For the formation of COP II vesicles some caot proteins and a GTPase is needed. Sar 1 is the GTPase here. The coat proteins are, Sec23/Sec24, Sec13/Sec31 and Sec 16. COP II is for the transport of protein from ER to golgi. Thus it starts with ER membrane. Sar1 in GDP bounded form binds with the integral membrane protein Sec12 on ER membrane. It causes the exchange of Sar GDP form into Sar GTP form. The Sar GTP thus binds with the ER membrane itself and recruits coat proteins- Sec23/Sec24 to it. it causes the vesicle formation, the vesicle formation is progressed with the recruitment of another coat protien complex Sec 13/Sec31 to the site. During the process membrane cargo proteins embedded in the vesicle act as signals to carry the cargo protein. After the vesicle is complete, the coat proteins have to be released. This is done by GTP hydrolysis of Sar 1 by Sec 23 subunit. Thus completed the COP II coated vesicle.

In Clathrin coated vesicles proteins are transported from plasma membrane and golgi to the endosome and golgi to the lysosome. The coat protein here known as calthrin. It is having 3 heavy chains and 3 light chains. It attaches to the membrane to form the vesicle. It is aided with another protein called adaptor protein (AP). The clathrin monomer get polymerised on the membrane with the associated AP complexex to complete the vesicle coat. The adaptor protein is here deciding the specific protein to be included in the vesicle. Thus different adaptor proteins are there for different cargo proteins. AP1 and GGA adaptors for the transport of proteins from golgi to endosome, AP2 for transport of proteins from plasma membrane to endosome. AP3 for transport from golgi to lysosome.

These are the elements required for COP II and Calthrin coated vesicles.