Proteins Reduce Surface Tension

macro photo of tan mushrooms against a blue background

Biological Strategy

Dikarya (Common Mushrooms, etc.)

Eleanor Banwell

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Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Fungi

Kingdom Fungi: Mildews, molds, mushrooms, rusts, smuts, and yeasts

Once thought to be more closely related to plants, fungi have been found to contain chitin in their cell walls, indicating a closer evolutionary relationship to animals. Fungi are decomposers, using digestive enzymes to break down their surroundings, making them an essential part of ecosystems worldwide, as new soil is formed from rotting matter. Almost all fungi reproduce through spores (mushrooms, molds) or asexual budding (some yeast). There are estimated to be more than 2 million species, but only about 140,000 have been described. Fungi have great importance in culture, food, and medicine (including being the source of penicillin) worldwide.

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Hydrophobin proteins from fungi reduce surface tension to enable growth by having both hydrophilic and hydrophobic parts.

For fungi, which grow at the micro- and nano-scale, the forces that influence them the most are different to the forces humans are used to managing. For example, gravity barely impacts fungi at these small scales, but the surface tension of water can over-power their attempts to grow.

Fungi grow in moist surroundings, and when they put forward new hyphae–tubular filaments through which they grow, feed and reproduce–they must break through films of water. Because, for hyphae, these forces are so large, all fungi from the subkingdom Dikarya produce special proteins called hydrophobins that reduce the surface tension of water. Dikarya includes many familiar fungi, such as edible mushrooms and yeasts.

Hydrophobins are small proteins that can dissolve in water but that have one highly water-repellent face consisting of hydrophobic amino acids. Because hydrophobins have both hydrophilic and hydrophobic parts, they are called amphipathic. The hydrophobic face repels water and this means hydrophobins tend to line up in a way that keeps that face dry. Hydrophobins naturally form films on surfaces, rod-shaped assemblies, or arrange themselves at the air-water interface. By doing this, they can keep their hydrophobic side away from water. When they arrange themselves at the air-water interface, hydrophobins disrupt the attractive forces between water molecules that give water such a high surface tension. Compounds that reduce surface tension in this way are called surfactants.

Fungi produce fruiting bodies that project up into the air. By releasing hydrophobins into the surrounding water layer, fruiting bodies are better able to break through the water surface and grow.

Image: Schor M, Reid JL, MacPhee CE, Stanley-Wall NR / CC BY - Creative Commons Attribution alone

Schor M, Reid JL, MacPhee CE, Stanley-Wall NR. 2016. The Diverse Structures and Functions of Surfactant Proteins. Trends Biochem Sci. 41(7): 610–620.

Image: Schor M, Reid JL, MacPhee CE, Stanley-Wall NR / CC BY - Creative Commons Attribution alone

Schor M, Reid JL, MacPhee CE, Stanley-Wall NR. 2016. The Diverse Structures and Functions of Surfactant Proteins. Trends Biochem Sci. 41(7): 610–620.

Image: Ccfarmer / CC BY SA - Creative Commons Attribution + ShareAlike

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References

“Arguably the most comprehensively studied group of surfactant and surface-active proteins is the fungal hydrophobins. First discovered in Schizophyllum commune, they have subsequently been found to be ubiquitous among ascomycetes and basidiomycetes. These fungi typically live in moist environments and erect aerial hyphae to spread. Under such conditions, surface tension at the air–water interface constitutes a significant barrier to hyphal erection. To overcome this barrier, the submerged hyphal cells secrete hydrophobins into the surrounding environment. The proteins assemble at the air–water interface, resulting in a significant decrease of the interfacial tension, which is essential for the hyphae to break through the surface. Moreover, they form a hydrophobic coating on the aerial hyphae, fruiting bodies, and spores. This coating not only allows the hyphae to colonize hydrophobic materials; it also facilitates dispersal of the spores into the air, promotes attachment to hydrophobic surfaces aiding invasion of hosts, and protects the spores of pathogenic fungi (e.g., Aspergillus sp.) from the host’s immune system” (Schor et al. 2016:610)

Journal article

The Diverse Structures and Functions of Surfactant Proteins

Trends in Biochemical Sciences, 41(7): 610–620 | 27/05/2016 | Schor M, Reid JL, MacPhee CE, Stanley-Wall NR

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Reference

“Hydrophobins are proteins produced by ﬁlamentous fungi that have very special properties; they are amphiphiles having hydrophilic and hydrophobic parts and are among the most surface active proteins known.” (Linder et al. 2005:878)

“…diﬀerent hydrophobins are expressed at diﬀerent stages of fungal life ranging from vegetative hyphae and sporulating cultures to the fruiting bodies (such as mushrooms).” (Linder et al. 2005:878)

“…hydrophobins can be found in the medium in liquid fungal cultures, they assemble onto the fungal cell walls, cover fungal spores, and coat the surface and air cavities in fruiting bodies.” (Linder et al. 2005:878)

Journal article

Hydrophobins: the protein-amphiphiles of ﬁlamentous fungi

FEMS Microbiology Reviews, 29(1): 877–896 | 01/11/2005 | Linder MB, Szilvay GR, Nakari-Setala T, Penttila ME

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