Overview of Proposed Model for ATP-Driven Protein Translocation Through SecYEG-SecA Complex (IMAGE)
Caption
In this figure, the protein is shown as surfaces, with SecA coloured in light blue and white, SecE coloured in light orange, SecG coloured in green and the two halves of SecY coloured in pink and grey. The bound nucleotide (ADP or ATP) is shown as orange, red and blue spheres. The surrounding lipid membrane is shown as orange spheres and wheat sticks. Shown as cartoon are the two-helix finger of SecA (blue) and the SecY lateral gate (pink and black). A simplified form of the translocating pre-protein is shown as small green spheres, with difficult-to-translocate regions (i.e. "blocks") shown as larger spheres. Left - in the closed state of the complex (with ADP bound), the pre-protein can diffuse freely back and forth unless a block is present. When this occurs, the two-helix finger of SecA 'senses' the block and triggers the release of ADP (yellow arrow). Right - the subsequent binding of ATP results in an opening of the SecY channel (yellow arrow; blue arrows), resolving the blocked pre-protein and once again permitting free diffusion (green arrow). After a set time, ATP is hydrolysed and the complex reverts to the closed state. If the blockage is still on the inside of the channel, the cycle is repeated. If the blockage is on the outside of the channel, then no mechanism exists to open the channel on the outside so the block has been 'ratcheted' through SecY.
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