As any chef knows, some liquids mix well with each other, while others do not. For example, when a tablespoon of grapes is poured into water, the movement will be short enough to carefully combine the two liters.
However, a tablespoon of oil poured into water comes together in droplets that cannot melt too much movement. The physics governing the mixing of liquids is not limited to the mixing of bowls; it also affects the behavior of objects within cells.
It has been known for several years that some proteins behave like liquids and that some liquid – like proteins do not mix together. However, little is known about how these liquid-like proteins behave on the cell surface.
“The separation between two non-mixing liquids, such as oil and water, is called‘ liquid-liquid phase separation ’, and is at the heart of the work of many proteins,” said Sagar Setru, 2021 Ph.D. a graduate who worked with both Sabine Petry, professor of molecular biology, and Joshua Shaevitz, professor of physics and the Lewis-Sigler Institute for Integrative Genomics.
Such proteins do not disperse inside the cell. Instead, they coexist alone or with a limited number of other proteins, allowing cells to share some biochemical activities without covering them within organ-attached spaces.
“In molecular biology, the study of proteins that form dense layers with liquid-like properties is a fast-growing field,” said Bernardo Gouveia, a graduate student of chemical and biological engineering, working along with Howard Stone, the ’69 Donald R. Dixon and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering, and chair of the department. Setru and Gouveia collaborated as first authors on an effort to better understand one such protein.
“We were aware of the behavior of the protein similar to TPX2 liquid. What makes this protein special is that it does not form melt droplets in the cytoplasm as seen before, but instead it appears that it differs in a degree from biological polymers called microtubules, “Setru said.
“TPX2 is essential for the formation of branched networks of microtubules, which is essential for cell division. TPX2 has also been overexpressed in some cancers, so its transport may have medical relevance. . “
Individual microtubules are rod-like linear filaments. During cell division, new microtubules form on the sides of the existing ones to form a branched network. The sites where new microtubules grow are marked by globules of thick TPX2. These TPX2 globules employ other proteins necessary to generate microtubule growth.
The researchers were curious about how TPX2 globules form on a microtubule. To find out, they decided to try to keep the process going. First, they modified the microtubules and TPX2 so that each one shone with a different fluorescent color.
Next, they placed the microtubules on a microscope slide, added TPX2, and then looked to see what would happen. They also observed at very high spatial resolution using a powerful imaging method called the atomic force microscope.
“We found that TPX2 first cooks the entire microtubule and then breaks up into droplets that are evenly spaced apart, similar to how the morning dew cooks nets. spiders and break up into droplets, “Gouveia said.
Setru, Gouveia and colleagues found that this is due to something that physicists call Rayleigh-Plateau instability. Although physicists may not recognize the name, they are already familiar with the miracle, which explains why a stream of water falling from a faucet breaks up into droplets, and why equal water cover on strands of spider web coming in separate beads.
“It’s amazing to find such everyday physics in the nanoscale world of molecular biology,” Gouveia said.
Extending their study, the researchers found that the extent and size of TPX2 globules on a microtubule is determined by the thickness of the original TPX2 coating – that is, the level of TPX2 present. This may explain why microtubule branching has altered in TPX2-overexpressing cancer cells.
We used simulations to show that these droplets are a more effective way of making branches than just getting a uniform coating or bonding of the protein throughout the microtubule. “
Sagar Setru, 2021 PhD Graduate, Princeton University
“That the physics of droplet formation, as clearly visible to the naked eye, plays a role at the micrometer blades, helping to establish the growing interface (no pun intended) between soft subject physics and biology, “said Rohit Pappu, Edwin H. Murty Professor of Engineering at Washington University in St. Louis. Louis, who was not involved in the investigation.
“The basic theory seems to be about a mixture of interface between liquid-like condensates and cell surfaces,” Pappu adds. “I suspect we will return to this work a- over and over again. “
Source:
Magazine Reference:
Setru, SU, et al. (2021) Hydrodynamic instability directs protein droplet formation on microtubules to accumulate branches. Physics of nature. doi.org/10.1038/s41567-020-01141-8.