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

Koreaki Ito,Naomi Shimokawa-Chiba,Shinobu Chiba

doi:10.12688/f1000research.21065.1

Abstract

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.

Highlights

In protein localization, hydrophobic segments of polypeptides play a central role by their ability to partition into the hydrophobic core of the membrane[1,2,3]
Hydrophilic parts of polypeptides must overcome the hydrophobic barrier of the lipid hydrocarbon to cross the membrane, whereas hydrophobic polypeptide segments should overcome the hydrophilic barrier of the phospholipid head groups to partition into the lipidic membrane interior
The YidC insertase provides an intramembrane platform that facilitates membrane insertion of a class of membrane protein We point out that the de novo integration function of translocon can be viewed as similar in concept to that of “insertases”, which occur in bacteria, mitochondria, chloroplasts, and eukaryotes

Summary

Introduction

Hydrophobic segments of polypeptides play a central role by their ability to partition into the hydrophobic core of the membrane[1,2,3]. In a well-known mode of transloconfacilitated integration, called stop-transfer[6,17], a hydrophobic segment of the polypeptide exits the channel laterally to reach the lipid phase of the membrane (Figure 2, panel 4).

Results

Conclusion