Question

Match the source of energy needed for unidirectional protein translocation across the membrane.

- Co-translational translocation into the endoplasmic reticulum (ER)       - Post-translational translocation into the ER       - Translocation into the mitochondrial matrix

A. ATP hydrolysis B. GTP hydrolysis

Answer 1

1) Co-translational translocation into the endoplasmic reticulum (ER) - GTP hydrolysis

ER signal sequence emerges from ribosome and attaches to SRP which then binds to SRP receptor on ER membrane==> the complex binds GTP==> transfer of new protein to translocon opens translocation channel to bring in the forming peptide, GTP is hydrolyzed, which leads to insertion of another polypeptide chain into the ER lumen.

2) Post-translational translocation into the ER - ATP hydrolysis.

The newly synthesized polypeptide chain is drawn through the translocon by an energy input from ATP hydrolysis by BiP in post-translational translocation.

3) Translocation into the mitochondrial matrix - ATP hydrolysis.

Translocation into the mitochondrial matrix occurs through a bipartite Tom/Tim complex in which Tom is the outer membrane translocon and Tim is the inner membrane translocon. Three energy inputs are required. First, ATP hydrolysis by a cytosolic Hsc70 chaperone keeps the newly synthesized mitochondrial precursor protein unfolded in the cytoplasm. Second, ATP hydrolysis by multiple ATP-driven matrix Hsc70 chaperones act to pull the translocating protein into the matrix.