Pheromone Gradient Tracking Mechanisms During Yeast Mating
Many cell types are remarkably adept at tracking chemical gradients, but they use different mechanisms in order to properly migrate or grow up-gradient. Bacteria use a temporal sensing mechanism to determine if they are swimming up-gradient. In contrast, eukaryotes are thought to use spatial sensing mechanisms where they compare the chemical concentration on one side of the cell to the other. In the present study, we utilized budding yeast <italic>(Saccharomyces cerevisiae) </italic>mating as a model for gradient tracking. Yeast cells are thought to use a spatial gradient tracking mechanism to grow up the pheromone gradient created by their mating partners. However, yeast cells polarize their receptors towards the direction of growth thereby reducing the distance that they can use to compare pheromone concentrations.
Yeast cells grow towards their mating partners by establishing a polarity patch that concentrates the master regulatory GTPase Cdc42 and its associated polarity factors on the membrane. The Cdc42 polarity patch orients actin cables so vesicles trafficking along these cables fuse at the polarity patch. Therefore, the location of the polarity patch determines the direction of growth. During mating, the pheromone gradient is thought to bias the polarity patch to the up-gradient side of the cell, but especially in shallow gradients, sometimes yeast cells initially establish the polarity patch on the wrong side of the cell. Work from our lab has found that the polarity patch wanders along the cell cortex during pheromone gradient tracking, and suggests that wandering behavior could serve as a mechanism of reorientation for cells whose polarity patch is misaligned with the gradient.
In order for yeast cells to properly track the pheromone gradient, their polarity patch must spend more time on the up-gradient side of the cell. How does the pheromone gradient bias wandering of the polarity patch to achieve this? We suggest that by polarizing their receptors and G proteins, yeast cells create a sensitized zone of the plasma membrane that can locally influence wandering of the polarity patch. As the polarity patch wanders along the cell cortex, so too does this zone of polarized receptors. If the patch wanders to a side of the cell with higher pheromone concentration, then more active receptors near the polarity patch could slow its wandering and allow more growth to occur in that direction.
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