6.2: Yeast Signaling
Yeasts are single-celled organisms, but unlike bacteria, they are eukaryotes—cells that have a nucleus. Cell signaling in yeast is similar to signaling in other eukaryotic cells. A ligand, such as a protein or a small molecule outside the yeast cell, attaches to a receptor on the cell surface. The binding stimulates second-messenger kinases (enzymes that phosphorylate specific substrates) to activate or inactivate transcription factors that regulate gene expression. Many of the yeast intracellular signaling cascades have similar counterparts in Homo sapiens, making yeast a convenient model for studying intracellular signaling in humans.
Signaling Cascades Drive Yeast Reproduction
Yeasts are members of the fungus kingdom. They use signaling for various functions, especially for reproduction. Yeasts can undergo “sexual” reproduction using mating pheromones, which are peptides—short chains of amino acids. Yeast colonies consist of both diploid and haploid cells. Both types of cells can undergo mitosis, but only diploid cells can undergo meiosis. When diploid cells undergo meiosis, the four resulting haploid cells, called spores, are not identical. In fact, the division of one diploid cell into four spores creates two “sexes” of yeast cells, each two cells of the type MAT-a and MAT-alpha.
MAT-a cells secrete mating signals called pheromones that attract the MAT-alpha cells, and vice versa. The mating pheromones bind to G-protein coupled receptors on the cell membranes. Upon binding, the G-protein initiates a mitogen-activated protein (MAP) kinase cascade. In this signaling cascade, a member of the MAP kinase protein family specifically phosphorylates another MAP kinase, which phosphorylates another, and so on. The kinases eventually phosphorylate transcription factors that alter the expression of nearly 200 genes to make the cell receptive to mating. These changes produce an elongation of the cell membrane and cytoplasm in the direction of the pheromone. This elongation is called a shmoo, and it continues following the pheromone concentration gradient until it connects with its mate. The two yeast cells merge, combining their chromosomes into a single diploid cell.
Quorum Signaling in Yeast
Even though yeast uses many cell signaling devices that are similar to the signaling mechanisms seen in more complex multicellular organisms, yeast is still individual cells, single-celled organisms, living in colonies, similar to bacteria. Like bacteria, yeast also uses quorum sensing signals between cells and even between colonies. At high cell density, yeast begins to secrete a quorum signal that aggregates individual yeast cells into colonies, with subcolonies expressing specialized functions or geographies. Quorum signals do not have to consist of complex molecules such as proteins. In fact, yeast can produce ammonia, a low-molecular-weight compound, which serves as a quorum signal that separates yeast cells into zones of viability and zones of apoptosis (programmed cell death). At the edges of a colony, where ammonia is least concentrated, cells proliferate. If enough ammonia is produced, it can inhibit the growth of the whole colony as well as the growth of neighboring colonies.
Yeast Can Infect Human Tissues
Yeast infections in humans include thrush (which forms painful white patches in the mouth), Candida esophagitis (thrush that spreads to the esophagus), vaginal yeast infections (causing vaginal pain, itching, and burning), skin itching and rashes including athletes’ foot (tinea pedis), and many others. The most dangerous form of yeast infection occurs when the fungus invades the bloodstream (fungemia). This is a life-threatening condition. All fungal infections spread when the fungi grow and signal to each other as described above.