Structure of the substrate-engaged SecA-SecY protein translocation machine

Area of Science:

• Molecular biology
• Cellular biology
• Biochemistry

Background:

• The Sec61/SecY channel facilitates protein translocation across membranes.
• SecA ATPase powers post-translational protein transport in bacteria via the SecY channel.
• The mechanism of polypeptide movement through the SecA-SecY complex remains unclear due to limited structural data.

Purpose of the Study:

• To elucidate the structural mechanism of protein translocation through the bacterial SecA-SecY complex.
• To visualize the polypeptide chain's path during translocation using high-resolution cryo-EM.
• To understand the role of SecA domains in polypeptide movement.

Main Methods:

• Electron cryo-microscopy (cryo-EM) was used to determine the structure.
• A translocating SecA-SecY complex was reconstituted in a lipid environment.
• Structural analysis focused on the interaction between SecA, SecY, and the translocating polypeptide.

Main Results:

• The cryo-EM structure captured a transition state of ATP hydrolysis during translocation.
• The polypeptide chain was visualized traversing both SecA and SecY components.
• SecA's two-helix finger and clamp domains were observed interacting with the polypeptide, with the clamp inducing a β-strand formation.

Conclusions:

• The structure provides a molecular basis for understanding protein translocation.
• SecA's two-helix finger and clamp domains likely cooperate to drive polypeptide movement.
• The findings support a model where SecA utilizes its domains in an ATP-dependent manner to translocate proteins through the SecY channel.