As the researchers peered closer into the cloud, they noticed that its magnetic field was curiously twisted. And then — as they examined a newborn star within the cloud — they spotted a hidden star, tucked behind it.

“It’s the star’s sibling, basically,” said Northwestern’s Erin Cox, who led the new study. “We think these stars formed far apart, and one moved closer to the other to form a binary. When the star traveled closer to its sibling, it shifted the dynamics of the cloud to twist its magnetic field.”

The new findings provide insight into binary star formation and how magnetic fields influence the earliest stages of developing stars.

Cox will present this research at the 240th meeting of the American Astronomical Society (AAS) in Pasadena, California. “The Twisted Magnetic Field of L483” will take place on Tuesday, June 14, as a part of a session on “Magnetic Fields and Galaxies.” The Astrophysical Journal also will publish the study next week.

Cox is a postdoctoral associate at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Twisted mystery

Stellar nurseries are wild and wondrous places. As dense clouds of gas and dust collapse to form stars, they launch outflows of stellar material at hypersonic speeds. A magnetic field surrounding a star-forming cloud is typically parallel to these outflows. When Cox and her collaborators observed the large-scale L483 cloud, they discovered just that. The magnetic field matched this typical profile.

But then the astrophysicists decided to take a closer look with NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), and this is when things became strange. The magnetic field was not, in fact, parallel to the newborn stars’ outflows. Instead, the field was twisted at a 45-degree angle, with respect to the outflows.

“At first, it matched what theory predicts,” Cox said. “If you have a magnetized collapse, then the magnetic field is controlling how the star is forming. We expect to see this parallelism. But theory can say one thing, and observations can say another.”

Unusual binary formation

Although more observations are needed, Cox believes a previously hidden sibling star may be responsible for the twisted field. Using SOFIA, the astrophysics team spotted one newborn star forming inside an envelope of material. But upon closer examination with radio telescopes at the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the researchers spotted the second star, sharing the same stellar envelope.

“These stars are still young and still forming,” Cox said. “The stellar envelope is what supplies the material to form the stars. It’s similar to rolling a snowball in snow to make it bigger and bigger. The young stars are ‘rolling’ in material to build up mass.”

About the same distance apart as our sun to Pluto, the two young stars form a binary system. Currently, astrophysicists agree that binaries can be formed when star-forming clouds are large enough to produce two stars or when the disc rotating around a young star partially collapses to make a second star.

But for the twin stars in L483, Cox suspects something unusual is at play.

“There is newer work that suggests it’s possible to have two stars form faraway from each other, and then one star moves in closer to form a binary,” Cox said. “We think that’s what is happening here. We don’t know why one star would move toward another one, but we think the moving star shifted the dynamics of the system to twist the magnetic field.”

Cox believes this new work ultimately could provide new insights into how binary stars — and the planets that orbit them — form. Most people are familiar with the iconic scene from “Star Wars,” in which Luke Skywalker wistfully gazes up at the binary stars that his home planet Tatooine orbits. Now, scientists know this scenario is not merely science fiction; planets orbiting binary stars potentially could be habitable worlds.

“Learning how binary stars form is exciting because planet and star formation take place at the same time, and binary stars dynamically interact with each other,” Cox said. “In our census of exoplanets, we know planets exist around these double stars, but we don’t know much about how these planets differ from the ones that live around isolated stars. With new instruments coming online to discover and probe new binary systems, we will be able to test these results with a statistical sample.”

The study, “The twisted magnetic field of the protobinary L483,” was supported by NASA and the National Science Foundation.

After it fills with cargo, the pit pinches off to form a clathrin-coated, membrane-bound vesicle inside the cell, which then proceeds to its proper destination. In cultured cells, hundreds of these clathrin-coated vesicles can form every minute.

However, conflicting models exist for how these vesicles assemble, and this has left a critical knowledge gap. One model is the constant-curvature model, where curvature is induced simultaneously with clathrin polymerization. Another, the flat-to-curved transition model, says that a flat lattice of clathrin molecules first assembles on the inside of the cell membrane, followed by a conformational change to form the clathrin-coated pit and vesicle.

Evidence for both models can be seen by electron microscopy — but those are static snapshots of fixed cells that do not reveal the actual nanometer-scale dynamics and possible pathways of clathrin-mediated endocytosis.

Now Alexa Mattheyses and colleagues at the University of Alabama at Birmingham and Emory University are filling that knowledge gap, using a sophisticated fluorescence microscopy imaging called STAR microscopy. This allowed them to follow clathrin-coated vesicle formation in living cells from initiation to completion, over periods up to 100 seconds.

Their study, reported in the journal Nature Communications, supports what has been called the flexible model of clathrin-coated vesicle formation, which includes both the constant-curvature and the flat-to-curved transition pathways.

“We show clathrin accumulation is preferentially simultaneous with curvature formation at shorter-lived clathrin-coated vesicles, but favors a flat-to-curved transition at longer-lived clathrin-coated vesicles,” said Mattheyses, an associate professor in the UAB Department of Cell, Developmental and Integrative Biology. “Together, our results provide experimental evidence in favor of the flexible model of endocytosis, where a single model of curvature formation cannot encompass the heterogeneity of cell-mediated endocytosis dynamics.”

Simultaneous Two-wavelength Axial Ratiometry, or STAR, microscopy is based on total internal reflection fluorescence, or TIRF, microscopy that allows visualization of fluorescently tagged proteins at or close to the plasma membrane, for a distance of about 100 to 200 nanometers. In STAR microscopy, the protein is labeled with two fluorophores that have different excitation wavelengths to take advantage of the wavelength-dependent penetration depth of the evanescent field. The result is an ability to simultaneously resolve both protein accumulation, as measured by fluorescence intensity, and distance from the plasma membrane, as measured by the fluorescence-intensity ratio of the two fluorophores. That distance, which represents the nanometer z-distribution, is a measure of curvature in the current study.

Using monkey kidney fibroblast cells, Mattheyses and colleagues labeled the clathrin CLCa light chain with fluorophores that have excitation wavelengths of 488 and 647 nanometers. They then stimulated the cells with epidermal growth factor to induce endocytosis. Quantitative analysis using the UAB Cheaha supercomputer resulted in 1,948 CLCa-STAR accumulations from 13 cells.

They found evidence of three models of curvature initiation: nucleation, where curvature formation started less than one second before clathrin appearance; constant curvature, where curvature formation started one to four seconds after clathrin arrival; and flat-to-curved transition, where curvature started more than four seconds after clathrin accumulation.

To analyze the data further, the researchers grouped endocytic events based on the clathrin lifetimes and the models of vesicle formation. This revealed interesting features.

“We found that short-lived endocytic events, less than 20 seconds, were formed predominantly through the constant-curvature model, while longer events — more than 20 seconds — favored the flat-to-curved transition,” Mattheyses said. “The proportion of nucleation events was smaller and favored short-lived events.”

To test whether this flexible model of clathrin-mediated endocytosis may be universal among different cell lines, the researchers also imaged human umbilical vein endothelial cells, stimulated by vascular endothelial growth factor to stimulate endocytosis. They found a similar range of endocytic events.

“These data show the previously unappreciated heterogeneity of clathrin-mediated endocytosis dynamics and plasticity of clathrin-coated vesicle formation, and they suggest different cell types or cargos can use this flexibility to impact the mode of curvature formation,” Mattheyses said. “Future research using STAR microscopy will define how the recruitment of accessory proteins and the role of biophysical parameters contribute to vesicle formation and how different clathrin dynamics impact signaling and cellular homeostasis.”

Co-authors of the study, “Imaging vesicle formation dynamics supports the flexible model of clathrin-mediated endocytosis,” are Tomasz J. Nawara, Yancey D. Williams II, Tejeshwar C. Rao and Elizabeth Sztul, UAB Department of Cell, Developmental and Integrative Biology; and Yuesong Hu and Khalid Salaita, Emory University Department of Chemistry, Atlanta, Georgia.

Mattheyses and Salaita developed STAR microscopy together at Emory in 2015. Mattheyses has been working with TIRF microscopy since her Ph.D. work at the University of Michigan with Daniel Axelrod, Ph.D., considered by many a pioneer of the technique.

Support came from National Institutes of Health grant GM3099, National Science Foundation CAREER grant 83200 and American Heart Association grant 906086.

Cell, Developmental and Integrative Biology is a department in the Marnix E. Heersink School of Medicine at UAB.

The method is so efficient that the team has calculated that waste yeast discarded from a single brewery in Boston would enough to treat the city’s entire water supply. Such a fully sustainable system would not only purify the water but also divert what would otherwise be a waste stream needing disposal.

The findings are detailed today in the journal Nature Communications Earth and Environment, in a paper by MIT Research Scientist Patritsia Statathou; Brown University postdoc and MIT Visiting Scholar Christos Athanasiou; MIT Professor Neil Gershenfeld, the director of CBA; and nine others at MIT, Brown, Wellesley College, Nanyang Technological University, and National Technical University of Athens.

Lead and other heavy metals in water are a significant global problem that continues to grow because of electronic waste and discharges from mining operations. In the U.S. alone, more than 12,000 miles of waterways are impacted by acidic mine-drainage-water rich in heavy metals, the country’s leading source of water pollution. And unlike organic pollutants, most of which can be eventually broken down, heavy metals don’t biodegrade, but persist indefinitely and bioaccumulate. They are either impossible or very expensive to completely remove by conventional methods such as chemical precipitation or membrane filtration.

Lead is highly toxic, even at tiny concentrations, especially affecting children as they grow. The European Union has reduced its standard for allowable lead in drinking water from 10 parts per billion to 5 parts per billion. In the U.S., the Environmental Protection Agency has declared that no level at all in water supplies is safe. And average levels in bodies of surface water globally are 10 times higher than they were 50 years ago, ranging from 10 parts per billion in Europe to hundreds of parts per billion in South America.

“We don’t just need to minimize the existence of lead; we need to eliminate it in drinking water,” says Stathatou. “And the fact is that the conventional treatment processes are not doing this effectively when the initial concentrations they have to remove are low, in the parts-per-billion scale and below. They either fail to completely remove these trace amounts, or in order to do so they consume a lot of energy and they produce toxic byproducts.”

The solution studied by the MIT team is not a new one — a process called biosorption, in which inactive biological material is used to remove heavy metals from water, has been known for a few decades. But the process has been studied and characterized only at much higher concentrations, at more than one part-per-million levels. “Our study demonstrates that the process can indeed work efficiently at the much lower concentrations of typical real-world water supplies, and investigates in detail the mechanisms involved in the process,” Athanasiou says.

The team studied the use of a type of yeast widely used in brewing and in industrial processes, called S. cerevisiae, on pure water spiked with trace amounts of lead. They demonstrated that a single gram of the inactive, dried yeast cells can remove up to 12 milligrams of lead in aqueous solutions with initial lead concentrations below 1 part per million. They also showed that the process is very rapid, taking less than five minutes to complete.

Because the yeast cells used in the process are inactive and desiccated, they require no particular care, unlike other processes that rely on living biomass to perform such functions which require nutrients and sunlight to keep the materials active. What’s more, yeast is abundantly available already, as a waste product from beer brewing and from various other fermentation-based industrial processes.

Stathatou has estimated that to clean a water supply for a city the size of Boston, which uses about 200 million gallons a day, would require about 20 tons of yeast per day, or about 7,000 tons per year. By comparison, one single brewery, the Boston Beer Company, generates 20,000 tons a year of surplus yeast that is no longer useful for fermentation.

The researchers also performed a series of tests to determine that the yeast cells are responsible for biosorption. Athanasiou says that “exploring biosorption mechanisms at such challenging concentrations is a tough problem. We were the first to use a mechanics perspective to unravel biosorption mechanisms, and we discovered that the mechanical properties of the yeast cells change significantly after lead uptake. This provides fundamentally new insights for the process.”

Devising a practical system for processing the water and retrieving the yeast, which could then be separated from the lead for reuse, is the next stage of the team’s research, they say.

“To scale up the process and actually put it in place, you need to embed these cells in a kind of filter, and this is the work that’s currently ongoing,” Stathatou says. They are also looking at ways of recovering both the cells and the lead. “We need to conduct further experiments, but there is the option to get both back,” she says.

The same material can potentially be used to remove other heavy metals, such as cadmium and copper, but that will require further research to quantify the effective rates for those processes, the researchers say.

The team also included Marios Tsezos at the National Technical University of Athens, in Greece; John Gross at Wellesley College; Camron Blackburn, Filippos Tourlomousis, and Andreas Mershin at MIT’s CBA; Brian Sheldon, Nitin Padture, Eric Darling at Brown University; and Huajian Gao at Brown University and Nanyang Technological University, in Singapore.