The invisibility cloak of a fungus

Model of the enzyme chitosan deacetylase (turquoise) with its substrate chitosan ( light green ) bound in its active site © Martin Bonin While viruses and bacteria regularly manage to infect the human organism, fungi only very rarely succeed. The reason for this is that the human immune system can recognize them very easily because their cells are surrounded by a solid cell wall of chitin and other complex sugars. Chitin is, so to speak, the alarm signal for our immune system, to which it reacts with a whole arsenal of defensive weapons. Some fungi, however, have apparently learned to avoid this fatal recognition: They possess one or more enzymes called chitin deacetylase, which they use to alter some of the chitin building blocks. This produces a chitosan, which is invisible to the immune system. A particularly aggressive fungus, Cryptococcus neoformans, which can easily lead to a fatal infection, particularly in immunocompromised patients, has four genes that appear to encode such enzymes. So far, however, it had only been possible to show that three of them really are chitin deacetylases.