Schematic diagram of the insertion of a membrane protein into a lipid bilayer cell membrane (structure with the light blue circles). On the left, the ribosome produces the new protein (red) and transfers it straight to the insertion machinery, which comprises a larger molecule complex. On the right, the new membrane protein can be seen in position inside the membrane.
Image Credit: © HHU / Alexej Kedrov
Scientific Frontline: Extended "At a Glance" Summary: Bacterial Membrane Protein Insertion
The Core Concept: Bacterial membrane protein insertion is the complex biochemical process by which newly synthesized hydrophobic proteins are transported from ribosomes and correctly folded into the cell membrane.
Key Distinction/Mechanism: Contrary to the long-standing belief that bacterial proteins enter the membrane exclusively through the "lateral gate" of the translocon, new research reveals they also utilize a "back-of-Sec" pathway. This mechanism was previously thought to exist only in the complex eukaryotic cells of higher organisms.
Major Frameworks/Components:
- Ribosomes: The primary cellular factories that synthesize nascent proteins within the aqueous interior of the cell.
- Insertases: Specialized enzymatic machinery, specifically the Sec translocon (SecYEG) and the helper protein YidC, responsible for receiving and embedding proteins into the lipid bilayer.
- Cryogenic Electron Microscopy: The high-resolution imaging technology utilized to determine the precise three-dimensional structure of ribosome-membrane protein complexes and visualize the complete insertion process.
Branch of Science: Biochemistry, Molecular Biology, Structural Biology, and Microbiology.
Future Application: A deeper structural understanding of these pathways can inform the bioengineering of synthetic membrane systems and potentially identify novel targets for antibiotic development by exploiting bacterial insertion mechanisms.
Why It Matters: This discovery overturns existing scientific paradigms by demonstrating that prokaryotes and eukaryotes share highly similar, evolutionarily conserved mechanisms for membrane protein insertion, fundamentally unifying our structural understanding of cellular biology.
Researchers from Heinrich Heine University Düsseldorf (HHU) have—in collaboration with colleagues from Ludwig Maximilian University (LMU) in Munich—analyzed the complex biochemical processes by which bacteria insert proteins into their cell membranes. In the scientific journal of the European Molecular Biology Organization (The EMBO Journal), 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 numerous proteins that 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 that detect control signals and trigger processes within the cell. These proteins are folded into complex, three-dimensional structures, and their specific forms are essential to protein function.
One question often remains unanswered for researchers: How do the proteins manufactured in the cell interior by the ribosomes—the "factories" of the cells—reach their positions within the membrane in the correct forms, and when did these processes become established over the course of evolution? Professor Alexej Kedrov, leader of the Synthetic Membrane Systems group at HHU, notes: "The environment inside the cell is very different from that of the membrane. In the aqueous environment in the cell interior, hydrophobic proteins would aggregate with other molecules before they could reach their target sites. Special insertion mechanisms are therefore necessary."
Nascent proteins are transported from the ribosomes to the membrane. Next, they are embedded in the membrane by special enzymes called "insertases," which include the Sec translocon and helper proteins such as YidC. It is only there that they achieve their final folded states. To date, researchers assumed that 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 recent studies on eukaryotes (higher cells with a nucleus), researchers have observed an alternative path into the membrane, wherein 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 Professor Kedrov examined the structure and insertion processes of proteins in bacterial cells, or prokaryotes. Kedrov states, "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 structures were then determined at LMU Munich using cryogenic electron microscopy. The researchers in Düsseldorf subsequently decoded the functionality based on these data.
Max Busch, a doctoral researcher in Professor Kedrov’s group and the lead author of the study, states: "For the first time, we have succeeded in showing the complete path from nascent membrane proteins in a ribosome to their insertion in the membrane. We have also seen when the three-dimensional folded structures of the proteins are formed."
These findings provide a better understanding of the folding processes of membrane proteins. "And we can learn something about the evolutionary development of these processes, which are important for the cells," emphasizes Kedrov, adding: "A similar process is also known to exist for other organisms, such as yeasts. We can thus deduce when this process became established during the development of living organisms and was preserved over the course of history."
An additional objective of Professor Kedrov's group is to examine membrane protein insertion in greater detail, building on the findings obtained to date. One particular focus will be clarifying the roles of the other proteins involved.
Published in journal: European Molecular Biology Organization
Title: Substrate-induced assembly and functional mechanism of the membrane protein insertase SecYEG-YidC
Authors: Max Busch, Cristian Rosales-Hernandez, Michael Kamel, Yulia Schaumkessel, Eli O van der Sluis, Otto Berninghausen, Thomas Becker, Roland Beckmann, and Alexej Kedrov
Source/Credit: Heinrich Heine University Düsseldorf | Arne Claussen
Edited by: Scientific Frontline
Reference Number: bchm062926_01
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