This paper explores the biology of yeast, deciphering the web of genes and proteins involved in a signaling cascade. It’s called Pheromone-Dependent Destruction of the Tec1 Transcription Factor Is Required for MAP Kinase Signaling Specificity in Yeast, and you can find it here. It tracks the cascade of chemical reactions that occur when a yeast cell detects a pheromone.
In a previous journal club series, we looked at quorum sensing as a way that bacteria can communicate with each other. Cellular communications don’t just happen externally, however, with cells of different species sending out signals out to warn or spy on their neighbors. Cells also need intracellular communication systems to respond to the outside world, grow, and maintain homeostasis.
A cell is a city. And inside it, lots of things are happening. DNA replication and cellular respiration are some of the chemical reactions known and loved by Bio 101 students. These kinds of processes go far beyond the glucose + energy –> carbon dioxide + water. It’s a multi-step process (think glycolysis, Krebs cycle, electron transport chain) that each involves its own cascade of enzymatic reactions. Proteins are needed each step of the way.
The result? A web of molecular reactions, summing up to a cell’s metabolism. Some things, like DNA helicase, are expressed when the cell needs to replicate its DNA prior to mitosis; they’re commonly found in the cell. In other cases, proteins are needed only in rare or particular circumstances.
In our paper today, we are looking at yeast cells (yes, the same kind of yeast used to bake bread. And that is the only connection I have to today’s photograph). The response we are looking at is whether they are in a mating phase or a filamentous (growth) phase. The stimulus is a pheromone; when filamentous yeast cells detect the pheromone, a signaling cascade is set off inside the cell, and they transition to mating phase.
In yeast, as in most non-bacterial cells, these cascades often use the same proteins to get the point across. Recycling the same proteins for different uses conserves resources, but it does increase the risk of cross-talk. How do cells keep all this information straight?
There are some things we already know before we need to start experimenting. We know that a protein called fus3 activates the mating response (when it detects pheromone). We also know that filamentation genes are controlled by the tec1 transcription factor. Loss of tec1 or fus3 results in crosstalk, and yeast no longer respond to the pheromone correctly; it continues filamenting when it should transition to mating response.
The first clue is shown in Fig. 2a. This figure is of a western blot, and it shows how much protein is expressed inside a cell. Tubulin is the positive control; yeast makes tubulin whether the pheromone is present or not. Add the pheromone, however, and over time, you see tec1 levels decrease. This indicates a degradation of tec1 as part of the mating response.
What happens next? And why is tec1 degradation so important for the cell to read the signal correctly?
Bao, M. Z., Schwartz, M. A., Cantin, G. T., Yates, J. R., & Madhani, H. D. (2004). Pheromone-dependent destruction of the Tec1 transcription factor is required for MAP kinase signaling specificity in yeast. Cell, 119(7), 991–1000. https://doi.org/10.1016/j.cell.2004.11.052