How do proteins get integrated into membranes?

What is it about?

Some of the newly synthesized proteins insert into a biological membrane and become integral membrane proteins. In more microscopic views, polypeptide parts enriched in hydrophobic amino acids partition into the lipid phase of the membrane. To facilitate the insertion events, cells contain "insertases," which provide intramembrane platforms that function as the halfway points of insertion. Cells also have the Sec translocon, which provides a transmembrane channel through which hydrophilic parts of polypeptide move to reach the other side of the membrane. This article clarifies how the Sec translocon is involved in protein insertion into the membrane.

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Why is it important?

The polypeptide-conducting channel of the Sec translocon is known to be involved in the insertion of a special type of hydrophobic segment of the client polypeptide, which leaves the channel via the lateral gate to become a transmembrane segment. However, other types of hydrophobic stretch that undergo "de novo" insertion into the membrane use the lateral gate directly (not from the channel interior), and such reaction should be catalyzed by insertase-like functions of the lateral gate region of the translocon. We present this proposal that seems to be supported by recent structural studies of the Sec translocon and the insertases.

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