Monday, May 21 , 2018, 7:02 pm | Fair 65º

 
 
 
 

UCSB Research Group Develops New Tool for Studying Membrane Protein Structure

Membrane proteins are responsible for transporting chemicals and messages between a cell and its environment. But determining their structure has proved challenging for scientists. A study by UC Santa Barbara's Han Research Group demonstrates a new tool to resolve the structure of membrane-embedded and membrane-associating proteins using the water dynamics gradient they found across and above the lipid bilayer as a unique ruler.

More than 25 percent of all human proteins are membrane proteins, which perform other essential functions, such as sensing and signaling. They also constitute approximately 50 percent of current drug targets, but the structure of only a small percentage has been resolved.

The UCSB study came about when researchers discovered that water on the membrane surface has a very distinct movement pattern: It is slowed down because the water is attracted to the membrane surface across several water layers. The scientists then wondered whether they could use this as an intrinsic ruler to determine how the associating protein is anchored into the membrane.

"It's very difficult to determine at what depth and in what conformation the protein is associating with the membrane, especially if you're talking about the interface or even the surface," said UCSB chemistry professor Song-I Han, the corresponding author of the study. "We found a contrast mechanism — water dynamics — which is distinctly different even above the membrane surface where there is no lipid density. The membrane surface distinctly changes the property of the water layers above it."

Postdoctoral scholar Chi-Yuan Cheng, the lead author of the study, and his colleagues used their unique spectroscopic tool to measure the water diffusivity at various positions of two membrane-associated proteins. These were carefully prepared by the Ralf Langen group at the University of Southern California, an expert team in the structure studies of membrane-associating proteins. The UCSB researchers then used the water dynamics gradient along the bilayer as an intrinsic ruler to determine structural information, such as topology, immersion depth and the location of the proteins, including the protein segments residing well away from the membrane surface — information that was previously unresolved.

"Membrane proteins can sit deep inside the membrane but also associate at the periphery of the membrane," Cheng said. "This can play a very important role in function, especially for peripheral and interfacial proteins."

"We are not suggesting our study can resolve the structure entirely, but it offers an important and missing puzzle piece to this problem," Han said. "While we may not be able to determine the entire structure, knowing the location and structure of a protein segment at the surface of membranes is very important."

The team used Overhauser dynamic nuclear polarization enhanced nuclear magnetic resonance (NMR), a technique they developed over the last few years. Using a small and stable radical with an even higher magnetic property than the hydrogen atom of water, researchers exploited this magnetic dipole by inserting it as a spin probe in the protein or membrane position of interest. They then used microwave irradiation to excite the dipole, which subsequently excited nearby water molecules, only when they moved at the same frequency as the dipole. In effect, the water near the spin probe was polarized, causing large NMR signal enhancements that could be measured to extract the local water dynamics.

"People have used this NMR relaxometry method before," Han said, "but what is novel is the fact the we use an electron spin as our excitation source, which has a much higher frequency than previously exploited, and that we actively drive the excitation of these spin probes with microwaves. The spin probes process at 10 gigahertz — instead of hundreds of megahertz — which allows us to look at the faster water dynamics relevant here as they are altered on biomolecular surfaces."

This proof of principal study is just the first step. Next the team will scrutinize neurodegenerative proteins, specifically tau, which plays a key role in Alzheimer's disease. The working hypothesis is that at some aggregation stages, tau may exert toxicity as it breaks through the membrane barrier, in part determined by its slowed surface water dynamics. The new tool developed by UCSB's Han Research Group can be used to test this hypothesis.

"You can imagine that the proof or disproof of the concept is hampered by the fact that it is very difficult to look at protein-membrane association," Han said. "Now we have a way of measuring these molecular assemblies. So if it's true that certain proteins species or their oligomers poke through, basically breaking down this barrier and making the membrane leakier, we surely should be able to see those features by looking at the surface water dynamics."

Support Noozhawk Today

You are an important ally in our mission to deliver clear, objective, high-quality professional news reporting for Santa Barbara, Goleta and the rest of Santa Barbara County. Join the Hawks Club today to help keep Noozhawk soaring.

We offer four membership levels: $5 a month, $10 a month, $25 a month or $1 a week. Payments can be made through PayPal below, or click here for information on recurring credit-card payments.

Thank you for your vital support.

Become a Supporter

Enter your email
Select your membership level
×

Payment Information

You are purchasing:

Payment Method

Pay by Credit Card:

Mastercard, Visa, American Express, Discover

Pay with Apple Pay or Google Pay:

Noozhawk partners with Stripe to provide secure invoicing and payments processing.

  • Ask
  • Vote
  • Investigate
  • Answer

Noozhawk Asks: What’s Your Question?

Welcome to Noozhawk Asks, a new feature in which you ask the questions, you help decide what Noozhawk investigates, and you work with us to find the answers.

Here’s how it works: You share your questions with us in the nearby box. In some cases, we may work with you to find the answers. In others, we may ask you to vote on your top choices to help us narrow the scope. And we’ll be regularly asking you for your feedback on a specific issue or topic.

We also expect to work together with the reader who asked the winning questions to find the answer together. Noozhawk’s objective is to come at questions from a place of curiosity and openness, and we believe a transparent collaboration is the key to achieve it.

The results of our investigation will be published here in this Noozhawk Asks section. Once or twice a month, we plan to do a review of what was asked and answered.

Thanks for asking!

Click Here to Get Started >

Reader Comments

Noozhawk is no longer accepting reader comments on our articles. Click here for the announcement. Readers are instead invited to submit letters to the editor by emailing them to [email protected]. Please provide your full name and community, as well as contact information for verification purposes only.

Daily Noozhawk

Subscribe to Noozhawk's A.M. Report, our free e-Bulletin sent out every day at 4:15 a.m. with Noozhawk's top stories, hand-picked by the editors.

Sign Up Now >