Transport Proteins Respond to Nutrient Concentration

Biological Strategy

Aspergillus nidulans

AskNature Team

Image: Sergio Gutierrez Getino / Shutterstock / Some rights reserved

Sense Chemicals (Odor, Taste, etc.) From the Environment

Chemicals are important for signaling and communication among living systems, either intentionally (such as when two living systems try to find one another) or unintentionally (such as when a plant emits a chemical signal that an herbivore can use to find a tasty bite). They are also important for other uses, such as navigating or finding sources for minerals. But chemical signals are often relatively weak and disperse when moving through water or gases. Therefore, detecting them requires specialized abilities, including a way to determine where they are coming from. A well known example of sensing chemicals can be seen in ants following a pheromone trail laid down by others in their colony to locate a quality and abundant food source.

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Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Regulate Cellular Processes

Cells are the basic building blocks of all living systems, so cellular processes dictate how physiological processes occur within those systems. Cells (whether entire unicellular organisms or parts of multicellular living systems) grow, metabolize nutrients (that is, chemically transform them), produce proteins and enzymes, replicate, and move. Cells as part of multicellular systems rarely act alone, instead having ways to signal to start and complete simple to quite complex interactions. How skin heals is a good example of the role of cellular processes. Blood cells called platelets release clotting factors to stop the bleeding; white blood cells rid the area of foreign materials and release molecules to coordinate healing; cells called fibroblasts start rebuilding using proteins called collagen; new blood vessels form; and skin cells called keratinocytes create the new surface.

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Capture, Absorb, or Filter Chemical Entities

Living systems often require chemical elements and chemical compounds, including complex sugars, proteins, and odor-making compounds, to perform critical activities. These compounds exist in various states–solid, liquid, and gas–and are ubiquitous in soil, water, and air. This requires that living systems not only have ways to capture, absorb, or filter them, but also ways to differentiate among them, selecting those that are valuable or harmful. For example, mangrove trees live with their roots in salty water and sediments. Various mangrove species have different strategies for removing salt from the water they take in so that their tissues can use the fresh water.

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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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A transport protein in Aspergillus nidulans maintains the right concentration of nitrogen compounds within cells by actively responding to the presence of nutrient sources.

Too much of a good thing is no good. Even microorganisms must know when to stop absorbing nutrients from their environment before they reach toxic concentrations. The fungus, Aspergillus nidulans, developed a clever strategy centered around a membrane-bound transport called UapA that functions to import nitrogen sources, including uric acid and xanthine, into the cell. Once the cell is satiated with enough uric acid and xanthine, UapA is sent to the cell’s recycling machinery where it’s broken down to its constituent building block s for use elsewhere. Curiously, it is the presence of uric acid and xanthine that trigger UapA to transport them into the cell in the first place, and it is these same compounds that mark UapA for recycling once the cell’s nitrogen requirements have been satisfied.

Uric acid and xanthine promote increased synthesis of UapA to facilitate their transport into the cell, but then induce break down of UapA once they are inside the cell. While the mechanisms are not yet fully understood, it appears that something inherent in the transportation of uric acid or xanthin across the cell membrane leads to subtle conformational changes in the shape of UapA, thereby signaling the cell to begin breaking it apart. It is attractive to hypothesize that the purpose of this unusual inhibitory pathway is to serve as a negative-feedback loop to prevent over accumulation of uric acid that may result in cellular damage. That may in fact be the case, but other pieces of evidence, including the low toxicity of uric acid to the fungus, shed doubt on the concept. As such, the precise reasons for this evolutionary remain unknown.