(L-R) Vlad Denic, Fei Wang, Matthew Tung
Eukaryotic cells target a large fraction of proteins synthesized in the cytosol to a number of different intracellular membrane compartments (organelles). Membrane targeting information is typically contained in hydrophobic signal sequences, which specify the organellar destination of proteins they are attached to. These molecular zipcodes are read out by specialized chaperones, called targeting factors (TFs), which tightly bind to them soon after they emerge from the ribosome. Upon encountering TF receptors on the cytosolic surface of organelles, TFs let go of their substrates, thus enabling them to be inserted into or translocated across organellar membranes. Our understanding of how TF receptors disrupt TF-substrate complexes is still rudimentary.
We used biochemical and structural approaches to address this conceptual problem in membrane protein targeting as it pertains to the cellular pathway for targeting proteins with C-terminal hydrophobic signals for insertion into the endoplasmic reticulum. We found that the TF receptor employs a dual mechanism for releasing substrates from the TF. First, it reels in TF-substrate complexes to the membrane using a long and unstructured region that it casts into the cytosol. Second, the TF receptor wedges and turns a rigid coiled coil between the two subunits of the TF dimer. This results in substrate release from the open TF dimer (followed by insertion of the hydrophobic signal into the membrane) because substrate binding is mediated by the juxtaposition of hydrophobic elements on both TF subunits. Once formed, the complex between the empty TF and the TF receptor is stable and makes the occupied TF receptor kinetically inaccessible to new TF-substrate complexes. However, ATP binding to the ATPase domain of the TF (but not hydrolysis or ADP binding) destabilizes the TF-TF receptor complex and primes the receptor-dissociated TF for a new round of substrate recognition. Precisely how substrates regulate this TF ATPase cycle to avoid futile consumption of metabolic energy in the absence of membrane targeting is an important future goal.
Read more in Molecular Cell
The Mechanism of Tail-Anchored Protein Insertion into the ER Membrane.
Wang F, Whynot A, Tung M, Denic V.
Structural basis for tail-anchored membrane protein biogenesis by the Get3-receptor complex
Stefer S, Reitz S, Wang F, Wild K, Pang YY, Schwarz D, Bomke J, Hein C, Löhr F, Bernhard F, Denic V, Dötsch V, Sinning I.
The cover that could have been.
[September 9th, 2011]