3D Structure of Sec61

CPK Color Scheme
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To the left is the Sec61 protein found in Saccharomyces cerevisiae. Sec61 is an essential part of the translocon complex in yeast and helps to facillitate the transport of newly translated proteins across the endoplasmic reticulum membrane. This protein is conserved across all forms of life and is necessary for the proper localization of proteins within the cell.

Reset and Spin Sec61

In order to transport soluble proteins across a lipid membrane, Sec61 forms an aqueous pore. This pore is formed by transmembrane regions 5, 6, 9 and 10, which are alpha helices with limited hydrophobicity (Wilkinson, et al., 1996). Select this button to highlight transmembrane region 5 (shown in green).

Select this button to highlight transmembrane region 6 (shown in yellow)

Select this button to highlight transmembrane region 9 (shown in orange)

Select this button to highlight transmembrane region 10 (shown in purple).

Sec61 is believed to form a heterotrimer. These four transmembrane regions (5, 6, 9 and 10) from each Sec61 moleculre present in the heterotrimer are predicted to come together to form the aqueous pore required for translocation. During active translocation, this pore has a diameter of 50 Angstroms.

Select this button to spin Sec61 to better see the transmembrane regions responsible for the formation of the pore.

In order for translocation to occur, the ribosome "carrying" the polypeptide chain that is awaiting translocation must be able to identify translocation pores that are not currently “in use.” A 2005 study by Cheng et al. indicates that the cytoplasmic loops six (L6) and eight (L8) serve as a recognition site for the ribosome during translocation

Highlight L6 (shown in pink).

Highlight L8 (shown in red).

Experiments by Cheng et al. determined that substituion of the arginine at residue 406 (located within L8) resulted in significantly decreased ribosomal binding. Select this button to spin Sec61 and highlight this crucial amino acid (shown in white).

Want to see how the complex looks in action? Click here to view the PDB file for the Sec complex bound to a ribosome.

Refrences:

Cheng Z., Jang Y., Mandon E., Gilmore R.  Identification of Cytoplasmic Residues of Sec61p Involved in Ribosome Binding and Cotranslational Translocation. Journal of Cell Biology 2005; 168.1: 67-77.

Wilkinson B., Critchley A., Stirling C. Determination of the Transmembrane Topology of Yeast Sec61p, an Essential Component of the Endoplasmic Reticulum Translocation Complex. The Journal of Biology Chemistry 1996; 271.41: 25590-25597.



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