Partially inserted nascent chain unzips the lateral gate of the Sec translocon

Many proteins in organelles of the secretory pathway, as well as secretory proteins, are translocated across and inserted into the endoplasmic reticulum membrane by the Sec61 translocon, a protein-conducting channel. The channel consists of 10 transmembrane (TM) segments of the Sec61α subunit and possesses an opening between TM2b and TM7, termed the lateral gate. Structural and biochemical analyses of complexes of Sec61 and its ortholog SecY have revealed that the lateral gate is the exit for signal sequences and TM segments of translocating polypeptides to the lipid bilayer and also involved in the recognition of such hydrophobic sequences. Moreover, even marginally hydrophobic (mH) segments insufficient for membrane integration can be transiently stalled in surrounding Sec61α regions and cross-linked to them, but how the Sec61 translocon accommodates these mH segments remains unclear. Here, we used Cys-scanned variants of human Sec61α expressed in cultured 293-H cells to examine which channel regions associate with mH segments. A TM segment in a ribosome-associated polypeptide was mainly cross-linked to positions at the lateral gate, whereas an mH segment in a nascent chain was cross-linked to the Sec61α pore-interior positions at TM5 and TM10, as well as the lateral gate. Of note, cross-linking at position 180 in TM5 of Sec61α was reduced by an I179A substitution. We therefore conclude that at least two Sec61α regions, the lateral gate and the pore-interior site around TM5, interact with mH segments and are involved in accommodating them.

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Sec translocon has an insertase-like function in addition to polypeptide conduction through the channel

F1000Research ◽

10.12688/f1000research.21065.1 ◽

2019 ◽

Vol 8 ◽

pp. 2126

Author(s):

Koreaki Ito ◽

Naomi Shimokawa-Chiba ◽

Shinobu Chiba

Keyword(s):

Essential Role ◽

Channel Model ◽

De Novo ◽

Lipid Phase ◽

Structural Studies ◽

Conducting Channel ◽

Sec Translocon ◽

Lateral Gate ◽

Insertion Process

The Sec translocon provides a polypeptide-conducting channel, which is insulated from the hydrophobic lipidic environment of the membrane, for translocation of hydrophilic passenger polypeptides. Its lateral gate allows a downstream hydrophobic segment (stop-transfer sequence) to exit the channel laterally for integration into the lipid phase. We note that this channel model only partly accounts for the translocon function. The other essential role of translocon is to facilitate de novo insertion of the N-terminal topogenic segment of a substrate polypeptide into the membrane. Recent structural studies suggest that de novo insertion does not use the polypeptide-conducting channel; instead, it takes place directly at the lateral gate, which is prone to opening. We propose that the de novo insertion process, in concept, is similar to that of insertases (such as YidC in bacteria and EMC3 in eukaryotes), in which an intramembrane surface of the machinery provides the halfway point of insertion.

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A molecular dynamics study on the resilience of Sec61 channel from open to closed state

RSC Advances ◽

10.1039/c9ra01684h ◽

2019 ◽

Vol 9 (26) ◽

pp. 14876-14883

Author(s):

Sujuan Sun ◽

Shuangshuang Wang ◽

Zhangfa Tong ◽

Xingdong Yao ◽

Jian Gao

Keyword(s):

Molecular Dynamics ◽

Chain Segment ◽

Closed State ◽

Nascent Chain ◽

Lateral Gate ◽

Sec61 Channel

The lateral gate of Sec61 is able to recover its partially-closed state rapidly after the nascent chain segment enters the bilayer, which triggers subsequent motions of the pore ring and plug.

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