Cell Biology of Microtubule Dynamics
Microtubule polymerization by TOG domain proteins
Although microtubules possess the inherent ability to form non-covalent polymers, a growing body of evidence implicates the XMAP215/Dis1 family of TOG domain-containing proteins as essential microtubule polymerases. We are studying the Drosophila homologue, Mini spindles (Msps), as a model for how these proteins regulate microtubule dynamic instability in vivo. Our data points to a mechanism in which Msps is able to associate with microtubule plus ends to promote microtubule assembly and convert to a lattice-bound pool to spatially regulate microtubule behavior in the cell periphery.
Mechanical crosslinking between actin and microtubules
Although the actin and microtubule networks are frequently studied as independent systems, they actually exhibit a high degree of crosstalk during processes such as cell division, migration, adhesion, and morphogenesis. This crosstalk can manifest as signaling pathways that allow actin and microtubules to locally influence each others dynamics, or as a mechanical cross-linkage between the two classes of cytoskeletal filaments. Although a number of molecules have been shown to link actin and microtubules, their cellular roles are unclear and their regulation in living cells is poorly understood. We are studying Drosophila Short stop (Shot) in order to understand how actin-microtubule crosslinking is regulated in the cell and to determine how it contributes to cell motility and morphogenesis. We found that Shot is required to maintain microtubule organization during interphase and that its cross-linking activity is required to resist deformative forces produced by microtubule motor proteins. In the absence of Shot, kinesin and dynein deform microtubules and cause them to exhibit exaggerated whip-like movements.
Microtubule severing by AAA proteins
Although the majority of microtubule growth and shrinkage occurs from the plus end, microtubule architecture is also influenced by a class of proteins that bind to the sides of microtubules and catalyze the formation of breaks in the tubulin lattice. These proteins, termed severing enzymes, play important roles in cell division, in neuronal outgrowth, and in determining cell shape in plants. Severing enzymes belong to the AAA ATPase superfamily - a functionally diverse group of enzymes that function as protein unfolding machines. Mutations in microtubule severing enzymes have been implicated in human diseases that contribute to neurodegeneration and various birth defects. Current projects in the lab are addressing how severing enzymes contribute to microtubule dynamics during cell migration and morphogenesis.