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Secretory vesicles, also known as dense core vesicles (DCVs), are membrane-bound vesicles that transport secretory proteins, such as hormones or neurotransmitters. Regulated secretory vesicles transport proteins from the trans-Golgi network to the exterior of the cell. Proteins present in regulated secretory vesicles are required to be rapidly exocytosed in large amounts upon a specific stimulus.
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Expression, Detergent Solubilization, and Purification of a Membrane Transporter, the MexB Multidrug Resistance Protein
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Structure and function of a membrane component SecDF that enhances protein export.

Tomoya Tsukazaki1, Hiroyuki Mori, Yuka Echizen

  • 1Department of Biophysics and Biochemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan.

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Summary

SecDF acts as a membrane chaperone, utilizing proton motive force to drive ATP-independent protein translocation across bacterial membranes. This protein complex facilitates protein transport and membrane protein biogenesis.

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Area of Science:

  • Bacterial protein secretion
  • Membrane protein biogenesis
  • Molecular mechanisms of transport

Background:

  • Protein translocation across bacterial membranes involves the SecYEG translocon and SecA ATPase.
  • Proton motive force and the membrane-integrated SecDF complex enhance this process.
  • The precise function of SecDF in translocation and membrane protein biogenesis remains incompletely understood.

Purpose of the Study:

  • To elucidate the structural and functional roles of the SecDF complex in bacterial protein translocation.
  • To investigate the mechanism by which SecDF facilitates protein transport.
  • To determine the contribution of proton motive force to SecDF-mediated translocation.

Main Methods:

  • Determined the crystal structure of Thermus thermophilus SecDF at 3.3 Å resolution.
  • Performed higher-resolution analysis of SecDF's periplasmic domains (P1 and P4).
  • Conducted in vitro translocation assays and electrophysiological analyses of SecDF.

Main Results:

  • The crystal structure revealed a pseudo-symmetrical transmembrane domain (RND superfamily) and periplasmic domains P1 and P4.
  • Periplasmic domain P1 undergoes conformational changes upon binding unfolded protein.
  • SecDF facilitates an ATP-independent translocation step requiring proton motive force and exhibits proton conductivity dependent on pH and substrate presence.
  • Conserved residues at the transmembrane interface are crucial for proton and preprotein movement.

Conclusions:

  • SecDF functions as a membrane-integrated chaperone.
  • Proton motive force powers an ATP-independent protein translocation mechanism mediated by SecDF.
  • SecDF plays a critical role in facilitating protein transport and potentially membrane protein biogenesis through proton conduction and conformational flexibility.