Nanoparticles Reveal the Structure and Function of Membrane Proteins

    Scientists at the Karolinska Institute have developed a nanoparticle technology that can be used to stabilize membrane proteins so that their structure can be studied in a lipid environment. The technique (“A Saposin-Lipoprotein Nanoparticle System for Membrane Proteins”), described in Nature Methods, makes it possible to access drug targets that previously could not be investigated and therefore potentially allows for the development of novel drugs, therapeutic antibodies, and vaccines, according to the team.

    “Our technology, termed Salipro, may offer a wide range of potential applications, ranging from structural biology to the discovery of new pharmacological agents, as well as the therapeutic delivery of protein-based therapeutics and vaccines”, says first author Jens Frauenfeld, who was working at the Department of Medical Biochemistry and Biophysics at Karolinska Institute when the study was performed.

    Membrane proteins are the targets of more than 60% of drugs in clinical use. In addition, the membrane proteins of viruses are the key functional unit in commercial vaccines. Thus, membrane proteins are important in biology, drug discovery, and vaccination. The problem that researchers face is that these proteins are very unstable and therefore hard to investigate. They are embedded in membranes that are made up of different kinds of lipids. Most often, detergents are used to extract the membrane proteins. However, detergents are associated with protein instability and poor compatibility with structural and biophysical studies. Moreover, detergents do not provide a lipid environment, which is important for membrane proteins.

    The investigators behind the new study worked around that problem by using the small cellular protein saposin. Usually, saposin shuttles lipids from one place to another within the cell.  Saposin is known to bind to lipids; thus, the researchers evaluated whether it would be possible to develop a method to make stable saposin-based lipid nanoparticles. They then expanded the method so that it is possible also to embed fragile membrane proteins into those lipid nanoparticles and stabilize them.

    The investigators demonstrated that the method facilitates high-resolution three-dimensional studies of membrane proteins by single-particle cryo-electron microscopy, cryo-EM, an increasingly popular technique among scientists who want to study proteins at atomic resolution. They also present a method to extract and stabilize fragile membrane proteins from the human immunodeficiency virus (HIV) virus membrane.

    “To our knowledge, the HIV spike protein preparation presented in the study using the Salipro system represents the first approach that allows the stabilization of the HIV-1 spike, including the important membrane domains, in a soluble and functional state,” says Prof. Pär Nordlund at the Department of Oncology-Pathology.

    The authors believe that the technology may also be applicable to other viral envelope proteins such as influenza virus hemagglutinin, Ebola virus G protein, or hepatitis C virus E protein. Taken together, the researchers apply the method on three different membrane protein targets for structural and functional studies.