Endoplasmic Reticulum

ID #2264

Protein folding can cause NLS to form. However for proteins destined to the mitochondria or RER, is the folding of a protein to display either their mitochondrial transfer sequence or their signal peptide not allowed because they must enter their destination in an unfolded or linear state?

Yes, you can think of it that way. An NLS and/or NES  on certain proteins becomes visible or recognizable once the protein has folded to its proper conformation. For proteins that are targeted to the mitochrondria, the transfer sequence at the N-terminus of the protein is presented in its linear state because the protein has to enter the TOM/TIM channels in a linear/unfolded conformation. For proteins destined to be sythesized in the RER, having the signal sequence/peptide (either N-ter or internal signal peptide) would temporarily stall translation because the SRP is going to bind to the signal peptide and the ribosome as it brings the entire complex to the translocon of the RER. So, without more of the protein made at this point, you couldn't have a fully folded protein present.

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