AsianScientist (Jun. 8, 2018) – To better appreciate the weird beauty of Phycomyces blakesleeanus, imagine you are about a micron tall and walking through a forest of this common fungus, related to the fuzzy mold that grows on forgotten food.
Looking up, you’d see long, thin stalks—fruiting bodies—towering above you, each crowned with a melanin-darkened, spore-filled sphere. If, indulging your curiosity, you used your knife to slice into the nearest stalk, out would pour a payload of octahedral—eight-faced—crystals. You don’t know what price this haul would fetch at the nearest micron-sized market, but the crystals are certainly of great value to the fungus—it uses them to sense gravity, and hence to point its fruiting bodies skywards.
But what are the crystals made of, and where did they come from? In a study published in PLoS Biology, researchers from Temasek Life Sciences Laboratory (TLL) in Singapore uncovered their surprising provenance: the crystals are encoded by a gene that the ancestors of P. blakesleeanus likely acquired from bacteria, and later repurposed into a gravity sensor.
Borrowing from bacteria
While scientists already understood that P. blakesleeanus distinguishes up from down by detecting how crystals settle within its cells, no one knew what the octahedral structures were made of. The TLL researchers, led by Dr. Gregory Jedd, managed to purify the crystals and identify their main building block: a protein named OCTIN. They then traced the evolutionary history of the gene that encodes OCTIN.
“We collected all of the OCTIN-related [gene] sequences we could find and then built phylogenetic trees that group the sequences based on how similar they are,” Jedd explained in an email interview with Asian Scientist Magazine.
To their surprise, they found that fungal OCTINs were most closely related to a group of sequences derived from certain bacteria, said Jedd. This suggests that at some point in the past, a fungal ancestor of P. blakesleeanus picked up the gene from bacteria through a process known as horizontal gene transfer, in which unrelated organisms can exchange genetic material.
Creativity with crystals
This finding was puzzling because bacteria aren’t known to sense gravity, said Jedd. This may in fact be dictated by a physical constraint—the TLL researchers estimated that bacteria, which at less than a micron in diameter are about one hundredth the thickness of a P. blakesleeanus stalk, would be far too small to employ crystals as gravity sensors.
Despite this, bacterial OCTIN is capable of self-assembling into nano-structures similar to the fungal crystals, as the researchers found out when they expressed it in Escherichia coli, a microbe which doesn’t naturally carry the gene for OCTIN. Thus, the bacterial protein, whose function remains unknown, already had properties that predispose it to gravity sensing, said Jedd.
Once the gene for OCTIN found its way into fungi, it accumulated mutations over time that changed the protein’s structure, allowing it to form bigger and bigger crystals. Eventually, these crystals became large enough for the fungi to use in gravity sensing, Jedd noted. In the genetic version of an IKEA hack, the fungus created a new function for an old, borrowed piece of DNA.
What fungi can teach us
There’s more to the story—because fungal OCTIN doesn’t form crystals in mammalian cell lines, Jedd thinks that there are other fungal molecules that are required for crystal assembly. His group is now searching for these factors, in the hopes of eventually being able to engineer nanoscale protein assemblies which could be used for applications such as drug delivery.
Fungi have fascinated Jedd his entire career, he said. Having first worked on budding yeast—a classical model system for understanding cell biology—he now uses multicellular fungi like P. blakesleeanus to investigate more complex traits like gravity sensing.
“Just like their yeast relatives, studying these more complex systems can teach us about fundamental aspects of how cells work, but they can also reveal insights into multicellularity and the evolution of complexity,” he said.
The next time you throw out a moldy object, spare a thought for the tiny organisms colonising it—they just might be doing something unexpected, like building crystal palaces to defy gravity.
This article is from a monthly column called The Bug Report. Click here to see the other articles in this series.
———
Copyright: Asian Scientist Magazine; Photo: Shutterstock.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.
