Cytoskeleton dynamics

Four of six talks in the Cytoskeleton Dynamics Minisymposium described work on filament end-binding proteins: a reflection of the central role they play in cytoskeletal organization. Scott Hansen's single-molecule study of purified vasodilator-stimulated phosphoprotein (VASP) demonstrated that it is a weakly processive actin polymerase that is targeted to filament ends by actin monomers. Hansen also described functional similarities between VASP and a microtubule polymerase, XMAP-215. The two proteins do not share an obvious common ancestor, suggesting that their similarities arose by convergent evolution. Magdalena Bezanilla, winner of the 2010 WICB Junior Award, described her work on the role of another actin polymerase, a formin, in polarized plant cell growth. In addition to promoting filament elongation, formins also comprise one of three known classes of nucleation factor that control where and when actin filaments are assembled in cells. Bezanilla showed that, in the moss Physcomitrella patens, one subclass of formins (class II) is a major driver of polarized cell growth. Unlike animal formins, class II plant formins are directed to their sites of activity by a PTEN domain, similar to that found in lipid phosphatases, which binds specifically to the acidic phospholipid PI(3,5)P 2. Sarah Gierke described her work on the role of microtubule end-binding proteins in epithelial migration and morphogenesis, showing how EB1, central adaptor protein of the " +TIP " complex, is required for early events of kidney tubule formation, including migration through extracellular matrix. Kaelyn Male, from Terry Lechler's laboratory, presented work on the adaptor protein desmoplakin, which recruits centrosomal proteins as well as the microtubule plus end–binding protein CLIP-170, to sites of cell–cell adhesion and forms the first step in a two-step process of microtubule reorganization required for epidermal differentiation. The remaining talks illustrate the diversity in the field. Tom Shemesh presented an in silico study of cytoskeleton and adhesion dynamics near the leading edge of migrating cells. This detailed mechanochemical simulation generated the interesting hypothesis that focal adhesions proximal to the leading edge act as screens that decrease mechanical forces felt by more distal adhesions. Because force affects assembly and disassembly of adhesions, this hypothesis may explain a key part of the mechanism underlying the life cycle of a focal adhesion. In the most divergent talk of the session, Jim Wilhelm reported that several metabolic enzymes in budding yeast form large polymeric structures, reminiscent of cytoskeletal filaments. Focusing on one, cytidine triphosphate (CTP) synthase, he found …