Question

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

Answer 1

most of the proteins into mitochondria are synthesised in cytosol and then transported to mitochondria. The proteins contain signal sequences. mitochondria is a double membrane organelle and protein import channels are placed in these membranes to facilitate protein transport. Proteins trasported to mitochondria are translocons.

PROTEIN TRANSPORT PATHWAY

Mitochodria have import receptor on outer membrane, recognize matrix targeting sequences or proteins. also there is inner mitochondrial membrane for transport. The protein to be transported have C terminus and N terminus ends where matrix targeting sequence is present at N terminus end. To ensure that the precursor proteins are unfolded during transport, cytosolic HSC 70 molecule has been attached which bind to the protein and keeps it unfolded. Only in unfolded state the proteins are transported through pre channels into mitochondrial matrix. This process requires energy in the form hydrolysis of ATP. HSC binds with import receptor and then transported to import channels of TOM 40 of outer membrane. To get transported to inner membrane, inner membrane contain translocons TIM 23/17 and TIM44. MATRIX HSC 70 pulls protein inside and helps in getting protein into mitochondrial matrix with energy from ATP molecule. now protein is inside matrix and MPP cleaves targeting sequence from rest of the protein molecule. Proteins starts folding into its functional form either with the aid of chaperonins or by autofolding. Finally the active protein is obtained.