Tip support: Microtubule stabilizing complexes in cells and in vitro

Abstract

To function properly, cells require a cytoskeleton consisting of four major polymeric networks. One of these networks consists of microtubules, which are hollow polymers involved in processes such as cellular transport and migration. Microtubules are dynamic, meaning they can switch rapidly between polymerization and depolymerization phases. In some cellular processes, this ability to switch is undesired and microtubules need to be stabilized. Stabilization can be accomplished by various proteins. Defects in these stabilizing proteins can manifest themselves in various diseases. In this thesis, we investigated the mechanisms controlling microtubule stabilization. We have studied two protein complexes involved in stabilization of growing microtubule ends. One of these complexes acts at the plasma membrane near focal adhesions. We found that in pancreatic cells, absence of one of the components of this complex, LL5β, resulted in attenuated insulin secretion. The other microtubule-stabilizing protein complex is located at the tip of primary cilia, where microtubules serve as physical support. This protein complex is involved in controlling the length of the cilia, and mutations in genes encoding components this complex are associated with conditions that affect multiple organ systems. We found that one component of the ciliary tip complex, CSPP1, stabilized slowly growing or damaged microtubules, while the combination of multiple components resulted in extremely slow microtubule elongation. Our results and future work may advance diagnosis and treatment strategies for the conditions linked to components of microtubule stabilizing complexes such as type 2 diabetes and the neurodevelopmental disorder Joubert syndrome.