Molecule of the Month: Ebola Virus Proteins

The genome of ebola virus contains instructions for building seven proteins, which assemble with the genomic RNA to form one of the deadliest viruses. Ebola virus is surrounded by a membrane stolen from an infected cell, and studded with ebola glycoproteins. A layer of matrix proteins support the membrane on the inside, and hold a cylindrical nucleocapsid at the center, which stores and delivers the RNA genome.

The ebola glycoprotein has the task binding to receptors on a cell surface and getting the ebola genome inside. It shares many of the features of other viral fusion proteins, such as influenza hemagglutinin and HIV envelope glycoprotein. It extends from the surface of the virus, and is covered with carbohydrate chains that hide it from the immune system. It is also a highly dynamic protein that snaps into a different shape when it binds to a cell surface, dragging the virus and cell close enough to each other that the membranes fuse. The structure shown here (PDB entry 3csy ) includes the receptor-binding portion of the glycoprotein and the machinery for membrane fusion. To allow cystallization, a small domain has been removed that includes much of the carbohydrate that normally covers the glycoprotein.

Ebola matrix protein, also known as VP40, shapes the virus and drives the process of budding. Many copies of the protein associate on the membrane, and are thought to make connections both to the membrane and to the nucleocapsid. The structure shown here (PDB entry 4ldd ) is a hexamer of subunits. A domain at the end of each chain is connected through a flexible linker--so flexible that four of them were not clearly seen in the crystallographic structure because they were moving so much.

At the center of the virus, a nucleocapsid composed of several types of proteins protects the genome. The ebola nucleoprotein wraps around the RNA, creating a helical complex. The interaction between nucleoprotein subunits, however, is not as rigid as in other viruses such as tobacco mosaic virus, so ebola virus often shows a wavy structure. Once inside cells, the large "L" protein, which is an RNA-dependent RNA polymerase, creates many new copies of the RNA genome.

As with the other ebola proteins, the nucleocapsid proteins contain several flexibly-connected domains, so researchers have studied them in parts. The RNA-binding portion of nucleoprotein has been studied by cryoelectron microscopy (PDB entry 5z9w), and x-ray crystallography has been used to study other parts of the protein (PDB entry 4qb0). Several other nucleocapsid proteins, which assist with formation of the structure, have also been studied (PDB entries 3vne, 3fke and 2i8b.)

Since viruses only have room in their small genomes to encode a few proteins, these viral proteins often moonlight in several jobs apart from their main task. The matrix protein of ebola is particularly unusual because it adopts entirely different structures for its different jobs: as a hexamer in the virion structure (PDB entry 4ldd ), as an octamer that binds to RNA and regulates viral transcription (PDB entry 1h2c ), and as a dimer involved in transport of the protein (4ldb ).