This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Researchers have been elucidating the structures of complex molecules for more than 100 years using a variety of techniques. Some molecular structures, however, are more difficult to solve than others.
Transmembrane proteins are massive molecules that span the lipid (fat) bilayers of the cell membrane to perform a variety of important functions, like controlling the flow of ions (charged atoms) into and out of the cell, detecting extracellular molecules, joining two cells together and accelerating chemical reactions. Seven-transmembrane (7TM) proteins, in particular, are large molecules, also known as G-protein coupled receptors, that bind to extracellular molecules and activate a multitude of different responses inside the cell. As their name suggests, these proteins have seven transmembrane regions that span the lipid bilayer of the cell membrane.
X-ray crystallography and cryo-electron microscopy (EM) have been used to determine high-level structures of 7TM proteins, but determining the high-resolution native structure of 7TM proteins in the lipid bilayer has been challenging. High-resolution structural information for molecules is typically generated using nuclear magnetic resonance (NMR) spectroscopy, a technique that uses a strong magnetic field and radio waves to determine the structure of molecules. To address this issue, a group of researchers from Yokohama National University, Bruker Biospin, Bruker Japan and The University of Tokyo developed a method to determine the high-resolution structure of schizorhodopsin SzR1, a 7TM protein proton pump.
Importantly, these results were obtained in a fully protonated form of SzR1, in which all amino acids that can accept a proton (H+) are filled, in a lipid bilayer, allowing the protein to maintain its native solid state. The team published their paper, "Ultrafast 160 kHz MAS enables high-resolution 1H-detected solid-state NMR of a fully protonated seven-transmembrane protein," on Aug. 11 in the journal Chemical Communications. "Membrane proteins are biologically important targets involved in cellular signaling and molecular transport, and their functions are closely linked to their structure and dynamics in the membrane environment.
However, strong 1H–1H [proton–proton] dipolar interactions broaden NMR signals, making it difficult to obtain detailed structural information without deuteration. We therefore aimed to determine whether ultrafast magic angle spinning (MAS) could overcome this limitation and enable high-resolution 1H-detected solid-state NMR of fully protonated membrane proteins in a native-like lipid environment," said Izuru Kawamura, professor in the Graduate School of Engineering Science at Yokohama National University and senior author of the research study. Deuteration is a process in which researchers swap normal hydrogen atoms (1H) with deuterium atoms (2H), a heavier and more stable isotope of hydrogen.
While this process can help with some of the signal problems associated with proton–proton dipolar interactions, deuteration is expensive, can cause changes in protein structure and can cause other signal degradation issues. Instead, the team used ultrafast magic angle spinning (MAS) NMR to determine the structure of the 7TM protein SzR1. MAS spins the sample at much higher speeds than normal NMR, in this case 160 kHz (or 160,000 cycles per second), with the sample oriented 54.74° relative to the magnetic field.
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