The Transport of Proteins into Mitochondria

Rohit BhardwajB.E. 2nd year Biotechnology

Delhi Technological University,Shahbad Daulatpur, New Delhi, INDIA.

All mitochondrial precursor proteins have a signal sequence at their N terminus that is rapidly removed after import by a protease (the signal peptidase) in the mitochondrial matrix.

These signal sequences are actually an amphipathic α helix, in which positively charged residues are clustered on one side of the helix, while uncharged hydrophobic residues are clustered on the opposite side. This configuration rather than a precise amino acid sequence is recognized by specific receptor proteins that initiate protein translocation.

Signal sequence on Mitochondrial proteins

These complexes contain some components that act as receptors for mitochondrial precursor proteins and other components that form the translocation channel.

The TOM complex. The TIM23 complex. The TIM22 complex The OXA complex

Protein Translocators in the Mitochondrial Membranes

Mitochondrial Precursor Proteins Are Imported into the Matrix at Contact Sites That Join the Inner and

Outer Membranes

Proteins transiently spanning the inner and outer mitochondrial membranes during their translocation into the matrix

TOM complex first transports the mitochondrial targeting signal across the outer membrane. Once it reaches in the intermembrane space, the targeting signal binds to a TIM complex, and the polypeptide chain either enters the matrix or inserts into the inner membrane.

Although the functions of the TOM and TIM complexes are usually coupled to allow protein transport across both membranes at the same time, both protein types of translocator can work independently.

Despite the independent functional roles of the TOM and TIM translocators, the two mitochondrial membranes at contact sites may be permanently held together by the TIM23 complex, which spans both membranes

ATP Hydrolysis and a H+ Gradient are Used to Drive Protein Import into Mitochondria

Mitochondrial protein import is fueled by ATP hydrolysis at two discrete sites, one outside the mitochondria and one in the matrix .

In addition, another energy source is required: an electrochemical H+ gradient across the inner mitochondrial membrane.

The requirement for hsp70 and ATP in the cytosol can be bypassed if the precursor protein is artificially unfolded prior to adding it to purified mitochondria.

Repeated Cycles of ATP Hydrolysis by Mitochondrial

Hsp70 Complete the Import Process

Thermal ratchet modelThe emerging chain slides back and forth in the TIM23 translocation channel by thermal motion. Each time a sufficiently long portion of the chain is exposed in the matrix, an hsp70 molecule binds to it, preventing further backsliding and thereby making the movement directional. Thus, a hand-over-hand binding of multiple hsp70 proteins translocates the polypeptide chain into the matrix.

Cross-bridge ratchet model The hsp70 proteins that bind to the emerging polypeptide chain undergo a conformational change, driven by ATP hydrolysis, that actively pulls a segment of the polypeptide chain into the matrix. A new hsp70 molecule can then bind to the segment just pulled in and repeat the cycle.

Protein import from the cytosol into the inner mitochondrial membrane or intermembrane space

The nuclear envelope

Nuclear Pores Perforate the Nuclear Envelope

Nuclear Import Receptors Bind Nuclear Localization

Signals and Nucleoporins

Possible paths for free diffusion through the nuclear pore complex.

Nuclear Import Receptors Bind Nuclear Localization

Signals and Nucleoporins

To initiate nuclear import, most nuclear localization signals must be recognized by nuclear import receptors, which are encoded by a family of related genes. Each family member encodes a receptor protein that is specialized for the transport of a group of nuclear proteins sharing structurally similar nuclear localization signals

The nuclear export of large molecules, such as new ribosomal subunits and RNA molecules also occurs through nuclear pore complexes and depends on a selective transport system. The transport system relies on nuclear export signals on the macromolecules to be exported, as well as on complementary nuclear export receptors. These receptors bind both the export signal and nucleoporins to guide their cargo through the pore complex to the cytosol.

The Ran GTPase Drives Directional Transport Through

Nuclear Pore Complexes

The compartmentalization of Ran-GDP and Ran-GTP.

A model for how GTP hydrolysis by Ran provides directionality

for nuclear transport

A model for how the binding of Ran-GTP might cause nuclear import receptors to

release their cargo