The Role of the Formin Protein Family in Membrane Dynamics

Using molecular genetics, and high end imaging techniques, I assessed the function of the formin protein family in the moss Physcomitrella patens. Formins are proteins that can nucleate and elongate actin filaments. P. patens has 9 formins, divided over three classes. I found that a class II formin (For2A) is essential for polarized growth and specifically binds to the phosphoinositide PI(3,5)P2. Additionally, I show that this formin polymerizes actin filaments in vivo. I demonstrated that binding PI(3,5)P2 is essential for formin function. My work also shows that one of the class I formins (For1F) is involved in exocytosis and likely is a part of the exocyst tethering complex, directly linking exocytosis to the actin cytoskeleton in plants. For1F is an essential gene, but its deletion can be rescued by overexpression of For1D, another class I formin, suggesting that class I formins are involved in exocytosis. Class I formins associate with actin filaments, but their interaction with actin differs from class II formin interaction with actin. Drug treatments show that their dynamics are dependent on both microtubules and actin filaments. This is in contrast to class II formins that do localize to endocytic sites and whose dynamics are only dependent on actin filaments. An endocytic marker can be seen traveling with the processive formin For2A when For2A is polymerizing an actin filament. Quantification of the activity of For2A along the length of tip growing cells reveals that For2A preferentially generates actin filaments towards the tip of the cell. This provides an actin array that is predominantly tip-oriented and could serve as a scaffold for myosins to transport cargo along towards the cell tip

Influence of electron beam irradiation on the microrheology of incompatible polymer blends : thread break-up and coalescence

The microrheology of polymer blends as influenced by crosslinks induced in the dispersed phase via electron beam irradiation, is systematically investigated for the model system polystyrene/low density polyethylene (PS/LDPE). Both break-up of threads and coalescence of particles are delayed to a large extent, but are not inhibited completely and occur faster than would be expected for a nonirradiated material with a comparable viscosity. Small amplitude, dynamic rheological measurements indicated that in the irradiated materials a yield stress could exist. In contrast, direct microrheological measurements showed that this yield stress, which would prevent both break-up and coalescence, could not be realized by EB irradiation. Apparently, the direct study of the microrheology of a blend system is important for the prediction of the development of its morphology and it is not possible to rely only on rheological data obtained via other methods

Melt rheology of electron-beam-irradiated blends of polypropylene and ethylene-propylene-diene monomer (EPDM) rubber

A detailed rheologicai analysis over large shear rate intervals has been performed for electron-beam-irradiated blends of polypropylene (PP) and ethylenepropylene-diene monomer (EPDM) rubber. At high frequencies, a lower viscosity results from irradiation compared with unirradiated blends, which implies that the irradiated blends are easily processable via injection molding. At low shear rates, however, the irradiated blends behave like a network, and the viscosity may even exceed the viscosity of the unirradiated blends. This particular behavior can result in the formation of weak weld lines. Aggregation of the dispersed, cross-linked EPDM particles into a skeletal structure is the most probable explanation. In a first attempt, it was tried to correlate the network behavior to the average (shortest) interparticle distance (ID) between two rubber particles, which takes into account both volume fractions and particle size of the dispersed phase. Provided that the EPDM rubber is sufficiently cross-linked, the network behavior becomes more pronounced; i.e., increase in viscosity with decreasing interparticle distance. Above a critical value of the ID, the viscosity does not change and is determined by the PP matrix. As a vast amount of literature indicates, the rheology of blends proves to be difficult to understand. Because of the more stable morphology, compared with usual blends, induced by irradiation, a more valuable interpretation of the rheological behavior is possible