Written and illustrated by: Madison Pletan
Edited by: Sarah Bassiouni, Claire Shudde, Sophie Maxfield, and Jennifer Baker
Imagine you’re a virus—a tiny shell of proteins surrounding some DNA or RNA. Your one mission is to infiltrate a living cell, navigate through its complicated compartments, and ultimately hijack it to make hundreds or even thousands of copies of yourself to perpetuate infection. As a virus, you are unable to multiply on your own. Since you lack the machinery to make new proteins and genetic material, you must enter a cell if you are to survive.
This is no simple task. For one thing, every cell is surrounded by a plasma membrane, a protective double barrier of fat that repels invaders. How can you, the virus, cross a membrane to access the inside of the cell?
Some viruses, called enveloped viruses, are surrounded by their own fat membrane, which they can fuse with the cell membrane to seamlessly slip inside. But other viruses do not have an envelope and therefore cannot fuse with the cell membrane to infiltrate the cell. They must instead devise a way to puncture or disrupt the cell’s membrane.
Which viruses infect cells by breaking through membranes? Everything from poliovirus to papillomavirus to polyomavirus and more—and no, these aren’t just a collection of scientific tongue-twisters! These viruses, along with many others, are considered nonenveloped viruses because they don’t have a fat membrane. Nonenveloped viruses are associated with a bevy of serious diseases: the ones listed cause polio, cervical and oral cancers, and kidney and nervous system disorders, respectively. Although each nonenveloped virus has a slightly different life cycle, each must find a way to break into the cytoplasm to establish infection. Using advanced microscope technology, scientists have recently been able to visualize how a particular family of viruses–reoviruses–accomplish such a difficult feat.
What are reoviruses, and how do they cross cell membranes?
Reoviruses are a diverse group of viruses that can cause many diseases in humans and animals, ranging from meningitis to diarrhea. You have probably experienced a mild reovirus infection, most likely in young childhood. Each individual reovirus is spherical, with three layers of protein making up the outer shell. Inside these layers, the virus has an RNA genome, which must be delivered to the cytoplasm of the infected cell for the virus to be copied.
To initially enter the cell, the reovirus undergoes a process called endocytosis, wherein a portion of the cell’s outer membrane buds inward and pinches off inside the cell to form a compartment with the virus inside. While now technically inside the cell, the virus is now trapped inside a tiny compartment—it can’t access any of the cell machinery it needs to multiply. To continue its infection, the virus must break across the protective membrane. Two research groups from UCLA and Harvard Medical School recently examined this process in unprecedented detail using cryo-electron microscopy. Cryo-electron microscopy is a super-resolution microscopy technique in which samples are flash-frozen and then bombarded with electrons, which allows the scientists to deduce the shape of molecules. In this case, they could use the technique to literally “see” the position of the viral proteins.
Researchers at UCLA used cryo-electron microscopy to study a reovirus called bluetongue virus, which primarily infects sheep and livestock. They found that bluetongue particles contain tightly folded proteins that use pH to detect when the virus buds into the cell. Once exposed to the acidic environment within the budded cell membrane, the viral proteins unfold outwards like tiny switchblades. In fact, the cryo-electron microscopy images of the folded and unfolded proteins were so detailed that the scientists could identify the tiny portion of the protein that senses the acidic environment, as well as the portion that shifts to cause the switchblade portion of the protein to dramatically extend. Further experiments showed that these extended proteins can pierce holes in the cell membrane, allowing viruses to escape and enter the cytoplasm.
Simultaneously, scientists at Harvard turned their electron microscopes towards a related virus called rhesus rotavirus. They asked the same basic question: how do these viruses penetrate the cell membrane? In a series of steps very similar to the bluetongue virus switchblade mechanism, one of the rotavirus outer proteins snaps outwards to embed itself in the cell membrane, eventually creating a hole for the virus to escape. The researchers validated their observation by creating a mutant version of the rotavirus penetration protein that was bound in place and unable to unfold, like adding a safety lock on the “switchblade”. This “locked” mutant virus was much less infectious than the normal strain—clearly demonstrating that free-moving penetration proteins are crucial for rhesus rotavirus to establish an infection.
How do other viruses cross cell membranes—and why should we care?
These two studies of reoviruses represent a major breakthrough in our understanding of how viruses penetrate and cross cell membranes. By applying cutting-edge microscope technology, scientists are able to reconstruct, in molecular detail, how viral proteins unfold to pierce holes in the membranes. This microscope technology may unlock more breakthroughs in the study of other nonenveloped viruses, whose methods of infection are more mysterious.
Studying the process of viral membrane penetration, therefore, has a clear, practical upshot: the better we understand this critical step of infection, the more effectively we can design drugs and therapeutics that block it. Put differently, we must study the weapons that viruses use to infect us, so we can incapacitate them. As a bonus, it’s just cool to witness the reovirus’s infinitesimally small outer proteins sense their environment, snap outward, and plunge into cell membranes. Who knew viruses could have such a sophisticated arsenal?
Madison is a PhD candidate in Cell and Molecular Biology at the University of Michigan. Her research focuses on how polyomaviruses hijack different cellular proteins to travel to the nucleus.
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