Читать книгу Astrobiology - Charles S. Cockell - Страница 133

It would be incomplete not to mention prions. These entities are made of misfolded proteins that can have disease-causing characteristics. One of the best characterized of these is the agent responsible for scrapie, one of several transmissible spongiform encephalopathies, which affect brain and neural tissue. Prions can induce normal proteins in cells to misfold into a stable configuration, which can then cause other proteins to misfold, thereby generating a chain reaction of misfolded proteins. These misfolded proteins are folded in such a way as to make them resistant to proteases, which are enzymes that normally break down defective proteins in cells. They have been reported in fungi. Like viruses, they can be reproduced in host cells, but they cannot reproduce by themselves, putting them outside most definitions of life. However, like viruses they invite us to advance our discussion of what operational definition we use for “life.”