* The nuclear membrane acts as a barrier: It controls what enters and exits the nucleus, where DNA is stored and transcribed into RNA.
* Proteins are made in the cytoplasm: Ribosomes, the sites of protein synthesis, are located in the cytoplasm, not the nucleus.
* Proteins are transported through nuclear pores: Proteins destined for the nucleus (e.g., transcription factors) are made in the cytoplasm and then transported into the nucleus through specialized channels called nuclear pores.
Here's a breakdown of the process:
1. Protein synthesis: Ribosomes in the cytoplasm create proteins according to the instructions encoded in mRNA.
2. Nuclear localization signal: Some proteins contain a "nuclear localization signal" (NLS), a specific amino acid sequence that acts like an address label, indicating the protein needs to be transported into the nucleus.
3. Importin binding: Importin, a protein chaperone, recognizes and binds to the NLS.
4. Transport through nuclear pores: The importin-protein complex travels through the nuclear pore complex, a structure embedded in the nuclear membrane.
5. Release inside the nucleus: Once inside the nucleus, another protein called RanGTP binds to importin, causing the complex to release the protein.
Therefore, proteins are transported into the nucleus, not from it.
