The function of the conserved scaffold protein Kel1 in regulation of cell morphogenesis, entry into mitosis and mitotic exit

How do cells control their shape, and how do cells respond to sudden changes in their environment? These are important biological questions, because many cells have highly specialised functions that requires them to take on a very specific shape. For instance, neurons can have extensions over 1m length, and macrophages need to be able to alter their shape to move through tissues to attack pathogens. Furthermore, sudden changes in the extracellular environment can harm cellular homeostasis and threaten survival of the organism. We are interested in unraveling the mechanisms that control cell shape and polarity, as well as the pathways that mediate cellular responses to stress. In this project we used the powerful model organism Saccharomyces cerevisiae (budding yeast) to identify novel regulators of these processes. We made the following discoveries: - The adaptor protein Kel1 is a major regulator of cell shape, particularly in response to extracellular cues. Here, Kel1 serves to help cells to interpret what is a true signal and what is merely background noise in a noisy environment. Briefly, we discovered that Kel1 is regulated by several kinases in response to internal and external cues, including the pheromone receptor. The pheromone receptor is a receptor that detects the presence of a mate of the opposite sex. When this occurs, the cell has to make an important decision: Arrest the cell cycle, find the mate, and undergo cellular fusion; or instead ignore the signal and proceed with the cell cycle to undergo cell division. This decision making process needs to be accurate, because the wrong decision can result in a competitive disadvantage relative to other cells in its environment. We discovered that unphosphorylated Kel1 induces the cell to ignore low-level signals that may occur as a result of background noise in the environment. However, activation of the pheromone results in activation of a kinase that phosphorylates Kel1, thereby resulting in full activation of the signaling pathway that triggers cell cycle arrest and cell fusion. Thus, we discovered a novel cellular decision-making mechanism. These discoveries are currently in preparation to be published. - Another type of decision-making is required when cells experience loss of their nutrient supply. In that case the cell needs to rewire its metabolic network, switching from anabolism to catabolism, and to switch to a state of energy conservation. One of the proteins that has an important function in the cellular stress response is the ubiquitin-like protein Sumo. We decided to study the role of Sumo in response to nutrient starvation, and discovered that Sumo is required for full activation of an anabolic transcriptional program. Thus, as long as nutrient levels were sufficient, Sumo was attached to several proteins that drive transcription of genes required for cell cycle progression and cell growth. However, starvation of cells resulted in changes in protein sumoylation that caused the cell to switch off these transcriptional programs, helping the cell to conserve energy. These findings were published in several articles and presented at multiple national and international conferences. Importantly, the NFR has recently granted a follow-up project that will study the response of cells to nutrient stress in much more detail.

One of the fundamental questions in modern biology is what determines the shape of an organism. Regulation of morphogenesis has been intensely studied in budding yeast. These studies revealed strong links between regulation of cell morphology and the cell cycle. Swe1 is an important factor in linking cell morphogenesis and the cell cycle. Swe1 is a kinase that phosphorylates and inhibits Cdk1, which is the master regulator of the cell cycle. However, the molecular mechanisms that control Swe1 to coordina te cell morphogenesis with cell cycle progression are very poorly defined. Building on our previous publications, we recently performed a high-throughput screen in budding yeast to identify proteins that regulate Swe1, and identified Kel1 as an novel reg ulator of Swe1. Kel1 is emerging as an important (but little studied) adaptor protein with poorly defined functions in cell morphogenesis, entry into mitosis and mitotic exit. This project is aimed at unraveling the functions of Kel1 in cell morphogenes is, entry into mitosis and mitotic exit. We present data showing that Kel1 directly binds Swe1, which may inhibit Swe1 activity. Furthermore, we found that Cdk1 can phosphorylate Kel1, which affects the localization of Kel1 and which impairs the interacti on between Kel1 and Swe1. In conclusion, Cdk1-Kel1-Swe1 may form a novel signaling pathway that regulates cell morphogenesis and cell cycle progression. This project is aimed at characterizing this novel molecular mechanism in detail.