Signaling in Single-Celled Organisms

Signaling in Yeast

Yeasts utilize cell-surface receptors, mating factors, and signaling cascades in order to communicate.

Learning Objectives

Describe how cell signaling occurs in single-celled organisms such as yeast

Key Takeaways

Key Points

  • Budding yeasts participate in a process that is similar to sexual reproduction that entails two haploid cells combining to form a diploid cell.
  • Budding yeasts secrete a signaling molecule called mating factor when trying to find another haploid yeast cell that is ready to mate.
  • In yeast, a cell signaling cascade is initiated when a mating factor binds to cell-surface receptors in other yeast cells.
  • A cell signaling cascade includes protein kinases and GTP-binding proteins that are similar to G-proteins.
  • Yeasts have 130 types of kinases, but they do not contain tyrosine kinases, which are utilized by multicellular organisms to control complex forms of development and communication.

Key Terms

  • kinase: any of a group of enzymes that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific target molecules (substrates); the process is termed phosphorylation
  • GTP-binding protein: a protein which binds GTP and catalyzes its conversion to GDP
  • G protein: any of a class of proteins, found in cell membranes, that pass signals between hormone receptors and effector enzymes

Signaling in Yeast

Yeasts are single-celled eukaryotes; therefore, they have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly-complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly.

The components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. Budding yeasts are able to participate in a process that is similar to sexual reproduction that entails two haploid cells combining to form a diploid cell. In order to find another haploid yeast cell that is prepared to mate, budding yeasts secrete a signaling molecule called mating factor. When mating factor binds to cell-surface receptors in other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins.

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Budding Yeasts: Budding Saccharomyces cerevisiae yeast cells can communicate by releasing a signaling molecule called mating factor. In this micrograph, they are visualized using differential interference contrast microscopy, a light microscopy technique that enhances the contrast of the sample.

Cellular Communication in Yeasts

Kinases are a major component of cellular communication. Studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms.

Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain similar counterparts to human signaling

Signaling in Bacteria

Bacterial signaling allows bacteria to monitor cellular conditions and communicate with each other.

Learning Objectives

Describe how cell signaling occurs in single-celled organisms such as bacteria

Key Takeaways

Key Points

  • Gene expression in bacteria is initiated when the population density of the bacteria reaches a certain level.
  • Bacterial signaling is called quorum sensing because cell density is the determining factor for signaling.
  • Quorum sensing uses autoinducers, which are secreted by bacteria to communicate with other bacteria of the same kind, as signaling molecules.
  • Autoinducers may be small, hydrophobic molecules, or they can be larger peptide-based molecules; regardless, each type of molecule has a different mode of action.
  • Some bacteria form biofilms, which are complex colonies of bacteria that exchange chemical signals to coordinate the release of toxins that attack the host.

Key Terms

  • quorum sensing: a method of communication between bacterial cells by the release and sensing of small diffusible signal molecules
  • autoinducer: any of several compounds, synthesized by bacteria, that have signalling functions in quorum sensing
  • biofilm: a thin film of mucus created by and containing a colony of bacteria and other microorganisms

Signaling in Bacteria

Signaling in bacteria, known as quorum sensing, enables bacteria to monitor extracellular conditions, ensure sufficient amounts of nutrients are present, and avoid hazardous situations. There are circumstances, however, when bacteria communicate with each other.

The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reached a certain level, specific gene expression was initiated: the bacteria produced bioluminescent proteins that emitted light. Because the number of cells present in the environment (the cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. Interestingly, in politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules, such as acyl-homoserine lactone (AHL), or larger peptide-based molecules. Each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off. The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.

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Autoinducers: Autoinducers are small molecules or proteins produced by bacteria that regulate gene expression.

Some species of bacteria that use quorum sensing form biofilms, which are complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that attack the host. Bacterial biofilms can sometimes be found on medical equipment. When biofilms invade implants, such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.

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Bacterial Biofilms: Cell-cell communication enables these (a) Staphylococcus aureus bacteria to work together to form a biofilm inside a hospital patient’s catheter, seen here via scanning electron microscopy. S. aureus is the main cause of hospital-acquired infections. (b) Hawaiian bobtail squid have a symbiotic relationship with the bioluminescent bacteria Vibrio fischeri. The luminescence makes it difficult to see the squid from below because it effectively eliminates its shadow. In return for camouflage, the squid provides food for the bacteria. Free-living V. fischeri do not produce luciferase, the enzyme responsible for luminescence, but V. fischeri living in a symbiotic relationship with the squid do. Quorum sensing determines whether the bacteria should produce the luciferase enzyme.