JdM recently asked Garn how viruses entered cells. Garn is still sulking after a not-entirely successful haircut, so I agreed to answer this question for him. Check out this Friday’s post for more of Garn’s adventures and the deets on crafty viruses here, below.
Let’s start with this diagram of a cell that you may remember from your biology classes.
Many cells are roughly spherical in shape and have an outer cell membrane which protects the cytoplasm of the cell in which the machinery of the cell is located. In the center is the nucleus, which holds the genetic material of the organism. What the diagrams don’t usually include is that the surface of cells aren’t smooth, impenetrable surfaces. The cell membranes are studded with all sorts of ion channels (used to regulate levels and intake of ions like sodium, calcium, and potassium within cells) and proteins called receptors. I spent many years studying a few different types of receptors, but never imagined I’d be blogging on them 😊.
The purpose of a cell receptor is to receive signals from outside of the cell. These signals can be small molecules like hormones, food for the cells (like glucose) or other cells and proteins. The surface of each receptor has a ‘pocket’ that is specific to the type of signal it is built for. An estrogen receptor would not react to another type of hormone, for example. Anyway – there are many types of cells in our bodies, and they all have different types of receptors.
When the signal docks into the binding pocket of the cell, a few things can happen: relay of signal, amplification, or integration into the cell.
The last option is what those crafty viruses use to enter cells.
Most viruses enter cells in similar ways, but we’ll use the novel coronavirus-2019 as an example. The surface of the novel coronavirus is studded with a Spike protein, which happens to be a good fit for the ACE2 receptor, which is found in respiratory cells. When the spike protein of the virus docks into the ACE2 receptor, the cell thinks a friendly neighbor (or food) has come to visit, so it invites the virus in by a process called endocytosis. Basically, the cell membrane fuses with and surrounds the virus and takes it inside the cell’s lysosome.
Once inside the enzymes in the cell helpfully disintegrate the virus’ coat, releasing the viral genetic material. From there, the virus uses the cell’s machinery to make more of the virus, and then the virus is packaged and exits the cells in a similar way to how it entered. The March 13 issue of Science has some pretty nifty diagrams of the entry process, here.
On Wednesday, I’ll share a little bit of how our body’s immune system and vaccines can help block this entry process and fight off the virus.
Stay safe and curious, my Friends!