No abstract

A protein has been purified from Acanthamoeba which, like cytochalasin B, caps the end of actin filaments normally favoured for monomer addition and inhibits the interactions of actin filaments. In addition, this 'capping' protein nucleates the polymerization of actin monomers and blocks the annealing of actin filament fragments.

No abstract

No abstract

We have identified a domain in the N terminus of huntingtin that binds to membranes. A three-dimensional homology model of the structure of the binding domain predicts helical HEAT repeats, which emanate a positive electrostatic potential, consistent with a charge-based mechanism for membrane association. An amphipathic helix capable of inserting into pure lipid bilayers may serve to anchor huntingtin to the membrane. In cells, N-terminal huntingtin fragments targeted to regions of plasma membrane enriched in phosphatidylinositol 4,5-bisphosphate, receptor bound-transferrin, and endogenous huntingtin. N-terminal huntingtin fragments with an expanded polyglutamine tract aberrantly localized to intracellular regions instead of plasma membrane. Our data support a new model in which huntingtin directly binds membranes through electrostatic interactions with acidic phospholipids. Huntingtin (htt)2 exists predominantly in the cytoplasm as a soluble protein that associates with the plasma membrane and multiple membranous organelles and vesicles (1-3). Many htt binding partners function in membrane trafficking (4). A specific molecular function for htt at membranes has not been demonstrated. The large size of htt (348 kDa) and interactions with numerous membrane-associated proteins suggest that htt may function as a scaffold.A polyglutamine expansion in the N terminus of htt (N-htt) causes neurodegeneration in Huntington disease (HD) and accumulation of htt in neurons. Degradation pathways for htt include endosomal-lysosomal and autophagic pathways and may require targeting to membranes to initiate clearance (5, 6). Therefore, knowledge of the effects of polyglutamine expansion on htt membrane targeting is important for understanding HD pathogenesis.N-htt has a membrane association domain. Membrane fractions prepared from control and HD brains (7) or prepared from cells expressing exogenous htt (5) contain N-htt fragments (ϳ440 -550 aa). In vitro translated normal and mutant htt (aa 1-548) localized to vesicles in extruded squid axoplasm (8). The structural features that mediate membrane association of N-htt are unknown and could include a proline-rich Src homology 3-binding domain that lies immediately distal to the polyglutamine stretch and HEAT domains, which are repeated regions of low homology shared by huntingtin with elongation factor 3, the p65 regulatory A subunit of protein phosphatase 2A, and TOR1 and a host of other proteins (9). The function of HEAT domains is unknown, but these repeated regions are leucine-rich and predicted to be ␣-helical in nature, with each repeat consisting of two helices and a short intervening linker. Crystallography studies of the protein phosphatase 2A p65 regulatory subunit (PR65/A) indicate that its multiple HEAT domains create a superhelical structure (10). htt has three domains at aa 205-329, 745-942, and 1534 -1710 containing a total of 10 HEAT repeats (9).In this study, we identify sequences in N-htt important for binding to membranes, explore mechanisms by which this bindin...