Early in its existence, Covid-19 gained a skill that would prove decisive in its relationship with human beings. The virus detected an apparently small change in its genetic code. It was probably an unfortunate accident – a fragment of genetic information from another virus was confused with that of the coronavirus while both infected a bat.
Included in this tiny piece of the genome, however, were instructions that altered a key part of the virus – its spike protein. This important protein penetrates the outside of the coronavirus and is the part that binds to the outside of cells, helping the rest of the virus to sneak in, where it can replicate.
This change in Covid-19’s spike protein meant it could hijack an enzyme found in the human body called furin. This enzyme acts like molecular scissors, usually cutting hormones and growth factors open to activate them. But when furin cuts off part of the Covid-19 spike protein, which is normally folded in a series of loops on the outside of the virus, it opens like a hinge.
“This exposes a new sequence in the peak protein,” says Yohei Yamauchi, a reader of viral cell biology at the University of Bristol, UK, who studies how this change may have caused Covid-19 to become more infectious in humans. “It is one of the changes that make this virus really different from the previous coronaviruses that caused Sars and Mers.”
This new mutation means that Covid-19 could suddenly attach itself to an important molecule found spread outside human respiratory cells called Neuropilin 1. This molecule helps to transport material within cells and more deeply into tissues – the mutation was like handing over the keys Covid-19 to a new port for our cells and meant that the virus could replicate in greater numbers in human airways.
Although this mutation was only one in Covid-19’s short existence, it proved to be important. Some researchers believe it may be one of the main mutations that allowed the coronavirus to skip species and start causing a disease that has spread rapidly in humans. But almost as soon as he did that, he began to pick up other mutations.