BIOC 212 Lecture Notes - Lecture 19: Ribosome, Translocon, Membrane Potential
29 May 2018
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8- Intracellular Trafficking
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TOM Complex
• Targeting across the outer membrane is carried out by TOM complex
• No cytosolic recognition proteins
o No proteins in cytosol that specifically recognize targeting sequences
§ First recognition is physically on the outer membrane itself
o Different from SRP and TRC40 mechanism
• TOM is starting point for sorting inside mitochondria
• Integral membrane receptors recognize signals on precursor
o Anchored at the outer membrane, and stick out into the cytosol --part in
cytosol recognizes the targeting signals
o For MTS (TOM20)
§ Recognize polypeptides and translocate them across outer membrane
§ Recognize MTS
o For IM internal signals (TOM70)
§ Specific for proteins going just into the inner membrane
• Import pore translocates proteins
o TOM40 aqueous pore, stabilized by other subunits
§ Receptors & TOM40 recognize polypeptides and translocate them
o Opening is regulated by insertion of precursor polypeptide into TOM40 pore
o Have affinity for polypeptide at receptors, and then TOM40 pore itself also
has affinity for precursor polypeptide
§ Has affinity for precursors itself
MTS Receptor
• TOM20 cytosolic domain recognizes hydrophobic side of an MTS
o Has a TM anchor and a domain in the cytosol that recognizes the
amphipathic MTS
o TOM20 has a hydrophobic groove that recognizes the hydrophobic side,
while the surface of TOM20 interacts with positive charges on MTS
§ Specifically recognizes MTS and not other amphipathic helices
• A series of affinity steps bring precursor across OM
o MTS will bind the TOM20 with a certain strength / affinity
o TOM20, the TOM40 pore and the TIM23 matrix-import complex have
increasing affinities for MTS
§ TIM23 complex in the inner membrane binds the MTS the strongest
• TOM20 binding increases probability of TOM40 binding, which in turn increases
probability of TIM23 binding
Document Summary
Tom complex: targeting across the outer membrane is carried out by tom complex, no cytosolic recognition proteins, no proteins in cytosol that specifically recognize targeting sequences. First recognition is physically on the outer membrane itself: different from srp and trc40 mechanism, tom is starting point for sorting inside mitochondria. Integral membrane receptors recognize signals on precursor: anchored at the outer membrane, and stick out into the cytosol --part in cytosol recognizes the targeting signals, for mts (tom20) Recognize polypeptides and translocate them across outer membrane. Recognize mts: for im internal signals (tom70) Specific for proteins going just into the inner membrane. Import pore translocates proteins: tom40 aqueous pore, stabilized by other subunits. Receptors & tom40 recognize polypeptides and translocate them: opening is regulated by insertion of precursor polypeptide into tom40 pore, have affinity for polypeptide at receptors, and then tom40 pore itself also has affinity for precursor polypeptide. Bring further into mitochondria: does not require energy input.
