Hungry yeast are tiny live thermometers

This fluorescence micrograph shows yeast vacuoles that have undergone phase separation. Credit: Luther Davis/Alexei Merz/University of Washington

Membranes are important to our cells. Every cell in your body is surrounded by one. Each of these cells contains specialized compartments, or organelles, that are also surrounded by membranes.

Membranes help cells perform tasks such as breaking down food for energy, building and breaking down proteins, tracking environmental conditions, sending signals, and determining when to divide.

Biologists have long struggled to understand exactly how membranes accomplish these different types of functions. The primary components of membranes – large lipid-like molecules called lipids and compact molecules such as cholesterol – form large barriers. In all but a few cases, it is unclear how these molecules help the proteins within the membranes to perform their functions.

In a paper published on January 25 in Proceedings of the National Academy of Sciences, a team from the University of Washington looked at phase separation in emerging yeast — the same single-celled fungi that is famous for baking and fermentation — and reported that live yeast cells can orchestrate a process called phase separation in one of their membranes. During phase separation, the membrane remains intact but integral into multiple, distinct regions or domains separating lipids and proteins. The new findings show for the first time that in response to environmental conditions, yeast cells precisely regulate the temperature at which their membrane undergoes phase separation. The team behind the discovery suggests that phase separation is likely a “switching” mechanism these cells use to control the kinds of work the membranes do and the signals they send.

“Previous work has shown that these domains can be seen in live yeast cell membranes,” said lead author Shantel Liveli, a doctoral student in chemistry at the University of Washington. “We asked: If it is important for a cell to have these domains, then if we change the cell’s environment — by growing it at different temperatures — will the cell take care and allocate energy to maintain phase separation in its membranes? The obvious answer is yes, it is.”

Previous research has shown that when sugar is plentiful, the yeast cell vacuole – an organelle important for storage and signaling – grows large and its membrane appears uniform under the microscope. But when the food supply dwindles, the vacuole undergoes phase separation, with many circular regions appearing in the organelle’s membrane.

In this new study, Leveille and her partners—UW professor of chemistry Sarah Keeler and UW professor of biochemistry Alexey Merz and Caitlin Cornell, formerly a doctoral student in chemistry from the University of Washington—sought to understand whether yeast can effectively regulate phase separation. Leveille yeast was grown at a typical laboratory temperature of 86 F with plenty of food. After the trophic diminution, yeast cell vacuole membranes underwent phase separation, as expected. When Leveille briefly raised the temperature in the yeast environment by about 25 degrees Fahrenheit, the spheres disappeared. Then Leveille grew the yeast at a lower temperature — 77 F instead of 86 F — and discovered that the spheres disappeared about 25 degrees above this new temperature. When the yeast was grown in still cooler conditions, at 68 degrees Fahrenheit, the phase separation disappeared again about 25 degrees above its growth temperature.

These experiments showed that yeast cells always maintain phase separation in the vacuole membrane until the temperature rises about 25 degrees above their growth temperature.

“We think this is a clear sign that yeast cells are engineering the vacuole membrane in different environmental conditions to maintain this consistent state of phase separation,” said Level.

She added that the phase separation in the vacuole membrane likely serves an important purpose in yeast.

“This result indicates that the membrane phase separation of yeast is likely a two-way door,” said Level. “For example, if the cells find food again, they will want to go back to their original state. The yeast doesn’t want to go too far from the transition.”

Future research could identify other membrane components that affect the ability of the gap membrane to phase separation, as well as the consequences of its phase separation. Biologists have known that when spheres appear in the yeast vacuole membrane, the cell stops dividing. These two events can be linked because the yeast vacuole membrane contains two protein complexes important for cell division. When the complexes are far apart, cell division stops.

“Proper vacuolar phase separation occurs when a yeast cell needs to stop dividing because its food supply is depleted,” Merz said. “One idea is that phase separation is the mechanism that the yeast cell ‘uses’ to separate these two protein complexes and stop cell division.”

In cells from yeast to humans, protein complexes in membranes influence cell behaviour. If additional research shows that phase separation in yeast vacuole regulates cell division, it is likely the first rigorous example of cell organization by this once-overlooked property of membranes.

“Phase separation could be a general and reversible mechanism for modifying many, many types of cellular properties,” Keeler said.

Cornell is now a postdoctoral researcher at the University of California, Berkeley.


Living cell membranes can self-sort their components by ‘de-mixing’


more information:
Chantelle L. Leveille et al, yeast cells actively adjust their membranes to separate phase at temperatures proportional to growth temperatures, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2116007119

Presented by the University of Washington

the quote: Hungry Yeast Are Small Live Thermometers (2022, Jan 25) Retrieved Jan 26, 2022 from https://phys.org/news/2022-01-hungry-yeast-tiny-thermometers.html

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