Yeasts are single-celled organisms, but unlike bacteria, they are eukaryotes (cells with a nucleus). Cell signaling in yeast is similar to signaling in other eukaryotic cells. A ligand, such as a protein or a small molecule released from a yeast cell, attaches to a receptor on the cell surface. The binding stimulates second-messenger kinases to activate or inactivate transcription factors that further regulate gene expression. Many of the yeast intracellular signaling cascades have similar counterparts in humans, making yeast a convenient model for studying intracellular signaling in humans.
Cell Signaling and Reproduction
Yeasts use signaling for various cellular processes including reproduction. They can undergo "sexual" reproduction between haploid mating cell types (MAT-a and MAT-α). MAT-a cells secrete mating signals called pheromones that attract the MAT-α 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 a cell protrusion in the direction of the pheromone. This protrusion is termed a shmoo, and it continues following the pheromone concentration gradient until a cell connects with its mate. Then, the two yeast cells merge to form a single diploid cell.
Quorum Sensing in Yeast Cells
In addition to reproduction, yeast cells use signaling for quorum sensing, a process commonly seen in bacteria, to monitor cell density and regulate behavior. For example, at high cell density, yeast begins to secrete a quorum signal that aggregates individual yeast cells into colonies. Yeast signaling also plays a vital role in pathogenesis and fungal infections in humans.
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