Mitochondrial and ER Protein Targeting: Signals, Complexes, and Translocation

What is the role of the mitochondrial localization signal?

It directs proteins to the mitochondria.

What complex facilitates the transport of proteins into mitochondria?

TOM/TIM complex.

What energy source is required for protein targeting to mitochondria?

ATP.

What state must proteins be in for mitochondrial targeting?

Unfolded.

What is the structure of the endoplasmic reticulum (ER)?

A continuous sheet from ER and nuclear membranes enclosing a single internal space.

What characterizes rough ER?

It contains membrane-bound ribosomes that coat the membrane.

What is co-translational protein import?

The process where proteins are translated and imported into the ER simultaneously.

What is the function of the N-terminal signal sequence in protein targeting?

It localizes the protein to the ER.

What amino acid composition is typical for ER-localized proteins?

A core of approximately 8 hydrophobic amino acids preceded by one or more basic amino acids (R/K).

What happens to the signal sequence of a mature protein in the ER?

It is cleaved off.

What is the role of the signal recognition particle (SRP)?

It guides the signal sequence to the ER and temporarily blocks protein translation.

What is required for the conformational change of SRP?

Energy from GTP.

What occurs during the recognition phase of protein targeting?

SRP binds to the signal peptide, pausing translation until it binds to the receptor.

What happens during the release phase of protein targeting?

Once the signal peptide is detected by the protein translocator, SRP is released.

What is the role of chaperone proteins like BiP-Hsp70 during protein translocation?

They assist in protein chain translocation and folding.

How does the process of co-translation improve efficiency?

Multiple ribosomal units co-translate one polypeptide unit.

What is the function of the Sec61 translocon?

It facilitates the transfer of the polypeptide chain into the ER lumen.

What is the stop transfer sequence?

An additional hydrophobic signal that halts translocation into the ER membrane.

What happens to the signal sequence during translocation into the ER membrane?

It is cleaved by signal peptidase.

What is the purpose of the lateral pore opening during protein translocation?

It releases the protein into the membrane.