Heimdall SNAREs reveal an ancestral architecture of eukaryotic SNARE complexes

Life as we know it is built on a fundamental divide between two types of cells. One type, found in bacteria and archaea, is simple and lacks internal compartments. The other, found in plants, animals, and fungi, is complex, filled with distinct rooms like the nucleus and the energy-producing mitochondria. For decades, scientists have wondered how this complex cell, with its intricate internal transport system, first emerged from a simpler ancestor. A major clue lies in the machinery that moves materials between these internal rooms. In complex cells, tiny bubbles called vesicles carry cargo from one place to another, fusing with target membranes to deliver their load. This fusion is driven by a specific set of proteins that act like molecular zippers, pulling two membranes together until they merge. These proteins, known as SNAREs, are essential for life in complex organisms, but their evolutionary origin has remained a mystery. Did this sophisticated machinery appear suddenly with the first complex cell, or did it evolve from a simpler system that existed in our ancient ancestors?

To answer this, researchers turned their attention to a group of microscopic organisms called Asgard archaea. Discovered relatively recently through genetic analysis of ocean sediments, these single-celled organisms are the closest known relatives to the ancestors of complex life. Some species within this group, specifically those called Heimdallarchaeota, have been found to possess genes that look surprisingly similar to those used by complex cells for internal transport. Among these genes are instructions for making proteins that resemble the SNARE zippers. However, having the genetic blueprint is not the same as proving the machine works. The critical question was whether these ancient proteins could actually assemble into the stable, four-stranded bundles required to fuse membranes, or if they were merely broken remnants of a system that never functioned in the way we see today.

A team of scientists set out to test this by isolating and studying these Heimdall SNARE proteins in the laboratory. They focused on a specific protein, which they named HPS1, found in a Heimdallarchaeota organism. First, they purified the protein and examined its physical properties. They discovered that the protein was incredibly stable, resisting heat and chemical stress that would destroy most other proteins. When they analyzed its shape in solution, the protein did not float around as a single strand; instead, it spontaneously grouped together with three identical copies of itself to form a tight, four-stranded bundle. This was a significant finding because, in complex cells, the functional bundle is made of four different types of proteins working together. The fact that the ancient version could form a stable bundle using four identical copies suggested a simpler, ancestral version of the machinery.

To see exactly how this bundle was built, the researchers grew crystals of the protein and used X-rays to map its atomic structure. The resulting image revealed a long, twisted rope made of four identical helices wrapped around each other. At the very center of this bundle was a specific layer where four identical chemical groups, made of the amino acid glutamine, met. In complex cells, this central layer is usually formed by a mix of different chemical groups, creating an asymmetric structure that is crucial for the fusion process. The researchers found that the ancient bundle was perfectly symmetrical, with all four sides looking exactly the same. Despite this symmetry, the core of the bundle was packed just as tightly and securely as the complex versions found in human cells. This indicated that the fundamental architecture of the SNARE bundle existed long before complex cells evolved, originating from a homogenous, four-identical-part system.

The researchers then used computer simulations to explore how this ancient bundle might have behaved compared to the modern version. They tested whether simply swapping one of the four identical glutamine groups for a different chemical group, as seen in complex cells, would immediately create the asymmetric structure found today. The simulations showed that this simple swap did not work; the bundle did not spontaneously rearrange into the complex, asymmetric shape. Instead, the ancient symmetrical design remained stable on its own. This suggests that the transition from the simple, identical four-part system to the complex, specialized four-part system was not a quick or accidental change. It likely required a longer evolutionary process where the proteins gradually changed and specialized over time.

Further experiments showed that when the researchers inserted the full-length version of this ancient protein into human cells, it successfully traveled to the cell's internal membrane system, specifically the endoplasmic reticulum. This demonstrated that the ancient protein was compatible with the cellular machinery of complex life, capable of inserting itself into membranes and finding its way to the correct location. The study also looked at the genetic history of these proteins, finding that they are present in various Heimdallarchaeota species but absent in other related groups. This distribution supports the idea that the ancestor of complex life likely possessed this specific type of SNARE protein before it split from its archaeal cousins.

The findings paint a clear picture of how cellular complexity may have arisen. The essential machinery for moving materials inside a cell did not appear out of nowhere when the first complex cell formed. Instead, it evolved from a pre-existing system in an ancient archaeal ancestor. This ancestor possessed a simple, symmetrical protein bundle that could form stable structures. Over millions of years, this basic toolkit was duplicated and modified, eventually giving rise to the diverse and specialized set of proteins that drive the complex internal transport systems of modern plants and animals. The study confirms that the deep roots of our cellular complexity lie in the ancient, symmetrical world of the archaea, where the first steps toward the intricate machinery of life were taken long before the first complex cell ever existed.