Proteins would clump together before reaching their target
At Heinrich Heine University Düsseldorf, researchers have discovered that bacteria navigate the delicate task of embedding proteins into their membranes through the same pathway once thought exclusive to more complex, nucleated life. The finding — that a so-called 'back-of-Sec' route is shared across the tree of life — suggests this molecular solution was forged early in evolution and preserved with remarkable fidelity across billions of years. In answering a seemingly narrow question about bacterial plumbing, science has glimpsed something older and more universal: a common inheritance written into the machinery of all living cells.
- A foundational assumption in cell biology — that bacteria and higher organisms insert membrane proteins through fundamentally different mechanisms — has been overturned by new structural imaging.
- The stakes are high: membrane proteins must fold into precise three-dimensional shapes to function, and misfolding means failure, making the insertion pathway a critical vulnerability and a potential target for drug design.
- Using cryogenic electron microscopy, researchers captured for the first time the complete journey of a nascent protein — from ribosome to translocon to its final folded position within the bacterial membrane.
- The discovery that bacteria use the same 'back-of-Sec' alternative pathway recently found in eukaryotes compresses the assumed evolutionary distance between simple and complex cellular life.
- The research team now plans to map the roles of additional proteins in this insertion process, pushing toward a fuller picture of how cells have solved one of their most ancient logistical problems.
A collaboration between Heinrich Heine University Düsseldorf and Ludwig Maximilian University Munich has quietly redrawn a boundary in cell biology. The question driving the work was fundamental: how does a newly made protein travel from the cell's watery interior to its rightful place in the membrane, without misfolding or clumping along the way?
Cell membranes are dense with proteins performing essential tasks — acting as channels, receptors, and signal detectors — each dependent on a precise three-dimensional shape. For decades, the accepted answer was that special enzymes called insertases, chiefly the Sec translocon, guided proteins through a lateral gate into the membrane. But imaging never quite confirmed this picture. Then, in studies of eukaryotes — organisms with nuclei — researchers spotted something unexpected: proteins were entering the membrane through the back of the translocon, a route no one had predicted. The question became whether this alternative pathway belonged only to complex life, or whether bacteria had been using it all along.
Professor Alexej Kedrov's team built ribosome-membrane protein complexes and sent them to Munich for cryogenic electron microscopy. The images were unambiguous: bacteria use the same back-of-Sec pathway. Doctoral researcher Max Busch described it as finally seeing the entire arc — protein birth, transit, and membrane arrival — in a single coherent picture.
The implications extend in two directions. For biotechnology and drug design, understanding how proteins fold during insertion opens new points of intervention. For evolutionary biology, the shared mechanism tells a deeper story: if bacteria and complex organisms rely on the same solution, it must have emerged early in life's history and proven so effective that billions of years of divergence left it essentially untouched. What began as a narrow question about molecular logistics has become a testament to how ancient and durable life's core inventions truly are.
A team of researchers at Heinrich Heine University Düsseldorf, working with colleagues from Ludwig Maximilian University in Munich, has upended a long-held assumption about how bacteria differ from more complex organisms. The question they set out to answer was deceptively simple: when a cell manufactures a protein, how does that protein actually get into the cell membrane where it needs to function? The answer, it turns out, is far more similar across the tree of life than anyone expected.
Cell membranes are studded with proteins that do essential work. Some act as channels, ferrying substances in and out. Others sit on the surface as receptors, waiting to detect signals and trigger responses inside the cell. These proteins are folded into intricate three-dimensional shapes, and that shape is everything—get the geometry wrong and the protein cannot do its job. The puzzle that has long vexed researchers is the journey these proteins must take. They are built by ribosomes, the cell's protein factories, in the watery interior of the cell. But that interior environment is fundamentally hostile to the proteins they are making. Hydrophobic proteins—those that repel water—would clump together with other molecules before they could ever reach the membrane. Something has to shepherd them there safely.
For decades, scientists believed they understood the mechanism. Proteins are transported from the ribosomes to the membrane, where special enzymes called insertases take over. The most important of these is the Sec translocon, often assisted by helper proteins like YidC. The prevailing model held that proteins entered the membrane through a single opening, a lateral gate in the translocon structure. But when researchers looked closely with imaging techniques, they could not actually see this happening. Then, in recent studies of eukaryotes—the higher organisms with nuclei—scientists observed something unexpected: an alternative pathway. Proteins were slipping into the membrane through the back of the translocon, a route no one had anticipated. The discovery raised an obvious question: Was this alternative route unique to complex cells, or did bacteria use it too?
Professor Alexej Kedrov's team decided to find out. They constructed ribosome-membrane protein complexes in Düsseldorf and sent them to Munich, where colleagues used cryogenic electron microscopy to map their structure in atomic detail. What they saw was striking. The bacteria were using the same back-of-Sec pathway that had just been discovered in higher organisms. For the first time, researchers had captured the complete journey: nascent proteins emerging from the ribosome, threading through the translocon, and settling into their final folded state within the membrane. Max Busch, the doctoral researcher who led the study, described the moment of clarity: they had finally visualized the entire path from protein birth to membrane insertion.
The implications ripple outward in several directions. Understanding how proteins fold as they insert into membranes opens new avenues for biotechnology and drug design. But there is also a deeper story here about evolution itself. If bacteria and higher organisms use the same insertion mechanism, that mechanism must have been established early in the history of life and then preserved across billions of years of divergence. The fact that yeasts also use this pathway suggests it became fixed at a particular moment in evolutionary time and proved so effective that nothing better replaced it. Kedrov's group plans to continue mapping these processes in finer detail, particularly the roles of other proteins involved in the insertion dance. What began as a simple question about bacterial plumbing has become a window into how life solved one of its most fundamental problems—and how that solution has echoed through time.
Citações Notáveis
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.— Professor Alexej Kedrov, Heinrich Heine University Düsseldorf
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.— Max Busch, doctoral researcher and lead author of the study
A Conversa do Hearth Outra perspectiva sobre a história
Why does it matter that bacteria and higher cells use the same protein insertion method? Doesn't every organism need to get proteins into membranes somehow?
True, but the assumption was that bacteria had evolved a completely different solution. If they had, it would tell us something about how much evolution can diverge. Instead, we're finding that they use the same trick. That suggests the trick is so good, so efficient, that once life found it, there was no reason to change.
But you said researchers couldn't actually see the lateral gate opening. So how did they know proteins were going through it?
They didn't. That's the whole point. The old model was based on logic and inference, not observation. When they finally looked with powerful enough microscopes, they saw something different—proteins taking a back route. And now they're finding that route in bacteria too.
What happens if a protein gets folded wrong during insertion?
It won't work. A misfolded protein can't perform its function. That's why the cell has all these special enzymes and helper proteins—to make sure the protein folds correctly as it goes in. It's not just about getting it there; it's about getting it there in the right shape.
Does this discovery change how we might design drugs or treatments?
Potentially, yes. If you understand exactly how proteins insert and fold, you can design molecules that interfere with that process in specific ways. You could target bacteria without harming human cells, or vice versa. But that's down the road. Right now, the researchers are still mapping the basic machinery.
So this mechanism has been around since early life?
Almost certainly. The fact that it's preserved in bacteria, yeasts, and higher organisms suggests it became established very early and worked so well that evolution kept it. That's rare. Most cellular machinery has been tinkered with extensively over time.