How proteins are inserted into cell membranes Lisa Lock Scientific Editor Andrew Zinin Chief Editor Researchers from Heinrich Heine University Düsseldorf (HHU) have—in collaboration with colleagues from Ludwig Maximilian University (LMU) in Munich—analyzed the complex biochemical processes that bacteria use to insert proteins into their cell membranes. They explain that—contrary to prior assumptions—there are more similarities between the processes in bacteria and higher cells than previously thought. The cell membrane contains a large number of proteins, which perform a wide variety of functions.

Some serve as transport channels, specifically guiding substances into the cell or carrying cellular products out of it. Others are receptors, which detect control signals and then trigger processes in the cell. These proteins are folded into complex three-dimensional structures, with the specific form essential for protein function.

One question had remained unanswered in many cases for researchers: How do the proteins manufactured by the ribosomes—the "factories" of the cells—in the cell interior reach their position within the membrane in the correct form, and when did the processes become established over the course of evolution? Professor Dr. Alexej Kedrov, leader of the Synthetic Membrane Systems group at HHU, said, "The environment inside the cell is very different from that of the membrane.

In the aqueous environment in the cell interior, the hydrophobic proteins would aggregate with other molecules before they could reach their target site. Special insertion mechanisms are therefore necessary." The nascent proteins are transported from the ribosomes to the membrane. In the next step, they are embedded in the membrane by special enzymes called "insertases," which include the so-called Sec translocon and helper proteins such as YidC.

It is only there that they achieve their final folded state. To date, it has been assumed that the insertion occurred exclusively via an opening in the translocon—the "lateral gate." However, it has not been possible to confirm this using imaging methods. In the latest studies on eukaryotes (higher cells with a nucleus), an alternative path into the membrane has now been observed, where membrane proteins are inserted via the back of the translocon ("back-of-Sec").

In the study now published in The EMBO Journal, the team headed by Kedrov examined the structure and insertion process of proteins in bacterial cells, so-called prokaryotes. "The recently published findings from eukaryotic systems have fundamentally changed our understanding of membrane protein insertion and challenged long-standing paradigms. This led us to ask the following key question: Is this newly described mechanism found exclusively in higher organisms, or does it also exist in bacteria?" To answer this question, ribosome-membrane protein complexes were produced at HHU, and their structure was then determined at LMU Munich using cryogenic electron microscopy.

The researchers in Düsseldorf subsequently decoded the functionality on the basis of these data. Max Busch, doctoral researcher in the group headed by Kedrov and lead author of the study, said, "For the first time, we have succeeded in showing the complete path from nascent membrane proteins in a ribosome through to their insertion in the membrane.