In-depth structural characterization of the tetraspanin CD81

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Every human cell is encased by a cell membrane that separates the cell contents from its surroundings. Proteins embedded in this membrane act as gates to allow molecules to enter and exit cells; they also mediate the interactions that occur between a cell and its environment. This means that membrane proteins are involved in many of the most fundamental processes in normal cell function; when these processes fail, diseases result. It is no surprise, then, that the top ten best-selling small molecule drugs of all time all target membrane proteins.There are many different membrane proteins in any given cell, grouped into over 1,500 families, each with many members. In order to study any of them in detail, it is important to understand their three-dimensional structures. Central to this is a technique called X-ray crystallography that allows scientists to obtain a detailed view of how the atoms within a protein are arranged, providing a framework for further study. Scientists use this framework to investigate how the protein functions, bringing new levels of understanding to how cells work in health and disease, and providing knowledge to develop new drugs.Tetraspanins are membrane proteins that function by interacting with a wide range of other membrane and soluble proteins, thereby affecting how cells signal, interact, change shape and move. Remarkably, tetraspanins are also involved in the process of infection for a wide range of diseases. However, because there is no known structure of any full-length tetraspanin family member, the mode of action of tetraspanins in these essential processes is not understood, leaving a major gap in our knowledge of cell biology.Obtaining the structure of any membrane protein is a major scientific challenge: It is necessary to remove the protein from the cell membrane which often results in the protein becoming so unstable that it cannot be used to make the crystals required to perform X-ray crystallography. Consequently, we know very little about many membrane protein families with important biological functions. We have now overcome this crystallization challenge for the tetraspanin, CD81.Human CD81 is one of the best understood tetraspanin family members and is the subject of our proposed research. It has well-established roles in how cells interact with each other, the immune response and fertilization. Notably CD81 is a receptor for some very important human pathogens including influenza, human immunodeficiency virus, the malarial parasite, T-cell lymphotropic virus type 1 and hepatitis C virus (HCV). It may also be a tumour promoter. Central to CD81 function (and to that of all tetraspanins) is its ability to form extensive interactions with itself and other proteins; however, we don't know what these structures look like and therefore lack the framework for further study, mentioned above.The first aim of the research outlined in our proposal is to solve the three-dimensional structure of CD81. We have made excellent progress towards this goal, having crystallized CD81 and collected X-ray diffraction data.We have also teamed up with scientists in France who can make soluble forms of the HCV protein, E2, that binds CD81. The second aim of our project is to make an HCV-E2/CD81 complex so we can characterize it and solve its structure; this will allow us to learn more about how CD81 interacts with other proteins. We believe we are the only team in the world that has all the tools to take on this challenge.Brand new developments in structural biology (e.g. high-resolution electron microscopy) have enabled us to devise a third aim, which is to look at these structures in the cell membrane (by electron tomography), linking our atomic level structural data to what is actually happening in the cell.Studying the structure of CD81 at this level of detail will allow us to begin to understand how tetraspanins work in health and disease.