Research

The organization and dynamics of the actin cytoskeleton are regulated by a plethora of proteins, which interact with monomeric and/or filamentous actin. The mechanisms that control actin filament nucleation and polymerization are relatively well established, whereas the principles underlying actin filament disassembly and recycling of actin monomers for new rounds of filament assembly are incompletely understood.

We apply a combination of structural biology, biochemistry and cell biology approaches to elucidate the molecular principles by which actin filaments are disassembled in various cellular processes, and how the newly depolymerized actin monomers are subsequently re-cycled to specific actin filament assembly machineries in cells.

We study, for example, how two evolutionarily conserved actin-regulators, twinfilin and cyclase-associated-protein, together with their various interaction partners, contribute to actin filament disassembly and monomer recycling. Altered expression levels of twinfilin and CAP are linked to many malignant cancers, and thus elucidating the cellular functions of these proteins will also provide insights into the mechanisms of cancer cell invasion and metastasis.

In addition to protrusive actin filament arrays that provide force for cellular processes involving membrane dynamics, most cell-types also harbor contractile actomyosin bundles. Here the force is generated through sliding of actin filaments past myosin II filaments. These contractile actomyosin bundles include e.g. myofibrils of muscle cells and stress fibers of non-muscle cells.

Stress fibers contribute to cell adhesion, morphogenesis and migration. Importantly, stress fibers are the most prominent mechanosensitive structures in many cells-types, and they only form on stiff matrix and align along external force in cells. However, the precise mechanisms by which stress fibers are assembled in cells, and how external forces and substrate rigidity control their assembly and alignment, are incompletely understood.

To uncover the general principles by which contractile actomyosin bundles are assembled in cell, we examine how various actin- and myosin-binding proteins regulate the mechanosensitive assembly and contractility of stress fibers in cells. Our studies also revealed that stress fibers are composed of several functionally distinct actin filament populations. Thus, among our specific interests is also to elucidate the assembly mechanisms and functions of these distinct actin filament populations.

Many actin-binding proteins bind membrane phosphoinositides, and their activities are regulated through these interactions. Typically, those proteins that induce actin filament assembly or link the actin cytoskeleton to the plasma membrane are up-regulated by phosphoinositides, whereas proteins promoting actin filament disassembly are inhibited by membrane phospholipids.

Consequently, an increase in the plasma membrane phosphoinositide (especially PI(4,5)P2 and PI(3,4,5)P3) levels promotes actin filament assembly at the membrane. Interestingly, many actin cytoskeleton-associated proteins, such as the BAR domain family proteins, can also directly sense and generate membrane curvature to induce plasma membrane protrusions and invaginations in collaboration with the actin cytoskeleton.

We use a wide array of biochemical, biophysical, and cell biological approaches to reveal how central actin-binding proteins associate with PI(4,5)P2-rich in vitro and in cells. Moreover, we study the physiological roles of BAR domain proteins, and examine how the membrane curvature sensing/generating activities of these proteins are linked to actin dynamics in cells.