Phase Transitions of Multivalent Adaptor Proteins
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Eukaryotic cells efficiently organize their activities to achieve their functional capabilities. This organization of biochemical reactions is a direct result of the cells' ability to compartmentalize their molecules. For example, within a eukaryotic cell, compartments like the nucleus, the endoplasmic reticulum and the vacuoles exist, which are relatively well known for their specific functions. These aforementioned compartments are surrounded by membranes. However, for the past hundred years, we have also known about assemblies of biomolecules that are not bound by membranes. After the initial discovery of nuages, other structures such as Cajal bodies, the nucleolus, promyelocytic leukemia (PML) bodies, paraspeckles, etc., were also described as membraneless organelles. Furthermore, membranes themselves are self-assembled entities of lipids, proteins and carbohydrates. Additionally, within and on surface of membranes, molecules cluster into signaling compartments in many different biological pathways. Interactions between individual biomolecules have been studied comprehensively in biology. One of our goals as biophysicists is to attempt to propose physical properties that allow these interactions at the subnanometer scale to give rise to formation of cellular structures, the compartments that are listed above. This thesis proposes a hypothesis based on polymerization of multivalent proteins that causes these complexes to phase separate in solution. The behavior of multivalent proteins and their ligands to phase separate may be a general property that allows cells to regulate their activities in certain localized compartments. To study this larger goal, I used a specific example of proteins involved in creating the slit-diaphragm, which is the filtration barrier of our kidneys. Nephrin, an integral membrane protein at the slit-diaphragm, interacts with its partners Nck and N-WASP in a multivalent fashion. I show here that these interactions create large assemblies that phase separate into liquid droplets, both in solution and on membranes. I also find that the creation of these assemblies affects the downstream biochemical activity of N-WASP toward the Arp2/3 complex and actin. The widespread existence of multivalent molecules suggests that these findings may have broad corollaries in different biological systems.