Viral spikes are proteins that stick out from the surface of an enveloped virus. These structures are not decorative; they are functional tools the virus uses to propagate. Each spike is a molecular machine designed for a specific purpose. The name “spike” comes from their appearance in electron microscope images, where they look like sharp projections from the viral body.
The Composition of Viral Spikes
Viral spikes are glycoproteins, meaning they are proteins coated with sugar molecules, a process known as glycosylation. These structures protrude from the viral envelope, the outer lipid membrane the virus acquires from its host cell. The protein part of the spike is anchored in this envelope, while the sugar-coated portion extends outward to contact host cells. The arrangement and density of these spikes give a virus its characteristic look.
A well-known example is the coronavirus family, named for the crown-like appearance created by its spike proteins. These spikes are large and composed of three identical protein chains that twist together to form a trimer. Different viruses have different spikes; for instance, influenza viruses have two types on their surface: hemagglutinin (HA) and neuraminidase (NA), each performing a separate role in the infection process.
The glycoprotein composition is important for the virus’s survival. The sugar coating helps shield the protein from the host’s immune system, camouflaging it from detection. These sugar molecules also help stabilize the spike structure in the body’s watery environment until it finds a cell to infect.
The Function of Spikes in Viral Infection
The primary role of a viral spike is to initiate infection by entering a host cell. The process is compared to a key fitting into a lock, where the spike protein is the key and a receptor on a host cell is the lock. This interaction is specific, meaning a virus’s spikes can only bind to certain receptors. This specificity determines which organisms or tissues a virus can infect.
For SARS-CoV-2, the virus that causes COVID-19, the spike protein’s receptor-binding domain (RBD) attaches to the angiotensin-converting enzyme 2 (ACE2) receptor on human cells. This binding triggers a series of molecular movements. Once attached, the spike protein changes its shape in a process called a conformational change.
This structural shift exposes a hidden fusion peptide, which inserts itself into the host cell’s membrane. This action builds a bridge between the viral envelope and the cell membrane, causing them to fuse. This fusion creates an opening for the virus to release its genetic material into the cell, allowing it to make copies of itself.
Viral Spike Mutations
Viruses, particularly RNA viruses, replicate quickly, and their replication machinery is prone to errors. These errors result in mutations in the genetic code for the spike protein. A mutation might alter a single amino acid in the protein chain, which can be enough to change the spike’s three-dimensional shape and function.
Some mutations have no effect, while others can be detrimental to the virus. Occasionally, a mutation provides an advantage. For instance, a change in the spike’s receptor-binding domain might make it bind more tightly to host cell receptors like ACE2. This enhanced binding can increase the virus’s infectivity, making it easier to spread from cell to cell and person to person.
Other mutations can alter the spike’s shape, helping the virus evade the host’s immune system. If antibodies from a prior infection or vaccination are shaped for a specific spike structure, a mutated spike may no longer be a match. This is known as immune evasion. The emergence of SARS-CoV-2 variants like Alpha, Delta, and Omicron illustrates this, as each had spike mutations affecting its transmissibility and immune recognition.
Targeting Spikes for Medical Intervention
Spike proteins are a primary target for medical interventions because they are on the virus’s exterior and initiate infection. The immune system recognizes these structures as foreign and produces antibodies in response. Some of these, known as neutralizing antibodies, are effective because they bind directly to the viral spikes.
This binding action can physically block the spike’s receptor-binding domain, preventing it from attaching to a host cell receptor. This natural defense is the principle behind many vaccines. For example, mRNA vaccines for COVID-19 provide the body’s cells with instructions to manufacture a harmless version of the SARS-CoV-2 spike protein.
The immune system recognizes these manufactured spikes and generates neutralizing antibodies and memory cells. If the person is later exposed to the actual virus, their immune system is prepared to use these antibodies to prevent infection. Beyond vaccines, antiviral drugs are also developed to interfere with the spike protein, either by blocking its binding or preventing the shape changes needed for cell entry.