Different proteins need to be in different areas on the cell, and the different areas of the cell are sectioned off as organelles with walls and membranes that block the way. This is why protein trafficking is a very important area of regulation in a cell. Proteins start from genes contained within the DNA of a cell, and this DNA is strictly kept inside of the nucleus. This DNA is transcribed inside of the nucleus into mRNA, which is what is able to leave the nucleus and give rise to proteins that need to circulate to the rest of the cell. This mRNA codes for signal sequences within its transcript that indicate where the coded protein needs to be transported, where it needs to be translated into a protein. These signal sequences can signal for the resulting protein to be imported into the endoplasmic reticulum, to be retained in the endoplasmic reticulum, to be imported into the mitochondria, and so on. The signal sequence is usually an area where a transport protein can bind and move the mRNA to an organelle while being translated. The protein that is translated usually remains unfolded until iside its organelle, except for proteins going to the nucleus. Fully folded proteins are able to go inside of the nucleus through a regulated pore. The signal sequence for nuclear import is called the Nuclear Localization Sequence (NLS). A protein called Importin binds to the NLS and binds to the Nuclear Pore Complex (NPC) of the nuclear membrane. This allows the Importin and protein to enter. Once inside, the protein Ran bound to GTP binds to Importin, causing a change in conformation that releases the protein into the nucleus. This GTP-Ran-Importin complex exits the nucleus.
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