To survive and fulfill their biological functions, cells need to take in material from their environment. In this process, proteins within the cell pull inward on its membrane, forming a pit that eventually encapsulates the material in a bubble called a vesicle.
Researchers from the University of Pennsylvania have now revealed a relationship that governs this process, known as endocytosis.
To survive and fulfill their biological functions, cells need to take in material from their environment. In this process, proteins within the cell pull inward on its membrane, forming a pit that eventually encapsulates the material in a bubble called a vesicle. Researchers from the University of Pennsylvania have now revealed a relationship that governs this process, known as endocytosis. To get around the limitations in what they could see through their microscopes, the developed model cell membranes. The membranes, labeled red, were partially sucked into a pipette. Membrane-bending proteins, labeled green, pulled the membrane out during endocytosis. By measuring the amount of membrane left in the pipette, they could track the relationship between cell tension and the amount of membrane-bending proteins at work.
Credit: University of Pennsylvania
Their new study, published in Nature Communications, shows that the threshold at which proteins succeed at making a vesicle depends on both the quantity of membrane-bending proteins and the tension in the membrane itself. As tension on the membrane decreases, fewer proteins are needed to reach that critical mass.
Calculating where this threshold is in a given cell would be useful for understanding many biological processes. Many diseases disrupt normal endocytosis, so altering this threshold might prove to be a basis for future treatments.
This relationship between protein activity and membrane tensions may also help explain the recently discovered "ultrafast endocytosis" pathway, in which cells are sometimes able to form a vesicle in a few milliseconds, thousands of times faster than usual.
The study was conducted by Tobias Baumgart, associate professor in the Department of Chemistry in Penn's School of Arts & Sciences, and Zheng Shi, a graduate student in Baumgart's lab.
Biochemists have identified a class of proteins that facilitate endocytosis by pulling on the cellular membrane. However, exactly what role each of the members of this class of proteins play and how many are needed to form a vesicle remains unclear. Microscopy techniques that have the resolution to answer such questions eliminate the possibility of observing the endocytosis in action.
"There are powerful techniques that allow scientists to clearly see how proteins change the membrane shape at a molecular level," Shi said. "However, it is more challenging to look at how this process changes with time with those techniques, because the samples usually must be brought into a solid state, such as by freezing them. For the same reason, it's also nearly impossible with these techniques to accurately control how tense the membranes are."
"Our approach," Baumgart said, "was based on a technique developed in our lab that can be used to control the membrane tension and look at the protein membrane-binding process in real time."
To get around the limitations in what they could see through their microscopes, the researchers developed a technique for inferring the necessary information from a model system.
They created stand-alone cell membranes, fluorescently labeled and partially sucked into a pipette. A standardized amount of suction drew a small amount of the membrane into the pipette, forming a "nose" on the otherwise spherical model cell.
The researchers exposed the model cell to a bath of membrane-bending proteins, fluorescently labeled in a different color. They attached themselves to the exterior of the model cell and began the endocytosis process in multiple places at once.
"From the perspective of the proteins," Baumgart said, "it doesn't matter that they're on the outside of the model cell. It's effectively a flat surface for them, just like the earth seems flat from our perspective."
As the proteins collectively pulled on the part of the model cell's membrane that was outside of the pipette, they were able to draw out some of the membrane that was trapped inside. This shortened the length of the "nose" left in there. By measuring this change, the researchers were able to infer the point at which endocytosis began to occur on the model cell's membrane. They could then calculate the total amount of protein involved at that point, as indicated by the intensity of their fluorescent markers.
Changing the strength of the suction pressure on the pipette also changed the overall tension on the model cells' membranes, allowing the researchers to directly observe the role tension played in regard to the number of membrane-bending proteins at work.
The interplay between these two factors means that the threshold at which endocytosis begins can be lowered not only by deploying more proteins but also by decreasing the overall tension in the membrane. Though it was not possible to study the entire mechanism in the model cells, the latter method might explain the speed with which ultrafast endocytosis begins in living cells.
"It's like getting a message to your friend by calling out rather than walking over," Baumgart said, "the tension signal literally propagates as a wave across the cell's membrane, which is much faster than making more proteins and needing them to physically get to the site where the vesicle is forming."
While their experiment only used one type of membrane-bending protein, future research using this technique will allow researchers to directly investigate the role other members of this class play in endocytosis.
The research was supported by National Institutes of Health grant GM 097552 and National Science Foundation grant CBET 1053857.
Evan Lerner | EurekAlert!
Hunting pathogens at full force
22.03.2017 | Helmholtz-Zentrum für Infektionsforschung
A 155 carat diamond with 92 mm diameter
22.03.2017 | Universität Augsburg
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
22.03.2017 | Materials Sciences
22.03.2017 | Physics and Astronomy
22.03.2017 | Materials Sciences