Question

Outline the steps involved in the import of a protein into the mitochondria. Indicate the steps that require energy.

Answer 1

First of all I want to outline that proteins are synthesized (fully folded protein) in both endoplasmic reticulum and cytoplasm. The protein that are syntheized in endocpasmic reticulum are secretory protein and protein synthesized in cytoplasm are transported to chloroplast, mitochondria and nucleus.

The polpeptide synthesized in cytoplasm from the messenger RNA have a signal sequence in their N- terminal region. That specific signal sequence is the mitochondiral localization signal. Now we know that mitochondria has both inner , outer membrane and mitochondrial matrix where different protein are present and these protein are imported to mitochondria from the cytosol. The signal sequences determines the location of the protein in mitochondria. so the signal sequences act as a specific address.

The polypeptide is transported from cytosol withthe help of a protein family called HSP (heat shock protein), specifically the cytosolic HSP 70 through the translocon present in the mitochondrial membrane.There are three types of translocon in mitochondria.

(1)TIM (translocse of inner membranc)

(2)TOM (tranlocase of outer membrance)

(3) OXA (oxidase assembly)

TIM are two types TIM 23 and TIM 22.

The TOM complex has receptor attached to the translocon which binds to the HSP protein present in outer mitochondrial membrance. the TOM needs the help of TIM complex to import in to the mitochondrial matirx The TIM- 23 translocator is bound to a receptor that spans through both inner and outer mitochondria and makes a  channel to pass the protein into matrix.

while ATP hydrolysis in cytosolic HSP 70 funnels the protein htrough these complex into the matrix. as the signal peptide enters into the matrix it got cleaved by peptidase. after that the mitochondrial HSP 70 attaches to the protein and drags the protein inside the matrix. this the second step for ATP hydrolysis.

For the protein to be transported into inner membrane two signal sequence are present, one for the mitochondrial matrix and other for embedding into the inner membrane. Here TIM -22 transfers several transmembrance protein into inner membrane and OXA does same for single transmembrane protein.