Lipid rafts and plasma membrane microorganization: insights from Ras

The spatial organization of plasma membrane components in discrete microdomains is thought to be a key factor in the generation of distinct signal outputs. A detailed characterization of plasma membrane microdomains, including descriptions of their size, dynamics and abundance, has proved to be a taxing problem for cell biologists and biophysicists. The use of novel techniques is providing exciting new insights into the challenging problem of plasma membrane microstructure and has allowed the visualization of domains with the characteristics expected of lipid rafts - microdomains of the plasma membrane enriched in cholesterol and sphingolipids. This review focuses on some of these recent advances and uses Ras signaling as a paradigm for understanding inner plasma membrane organization and the role of lipid rafts in cellular function.

Neuromuscular adaptation during skill acquisition on a two degree-of-freedom target-acquisition task: Dynamic movement

In this experiment, we examined the extent to which the spatiotemporal reorganization of muscle synergies mediates skill acquisition on a two degree-of-freedom (df) target-acquisition task. Eight participants completed five practice sessions on consecutive days. During each session they practiced movements to eight target positions presented by a visual display. The movements required combinations of flexion/extension and pronation/supination of the elbow joint complex. During practice sessions, eight targets displaced 5.4 cm from the start position ( representing joint excursions of 54) were presented 16 times. During pre- and posttests, participants acquired the targets at two distances (3.6 cm [36 degrees] and 7.2 cm [72 degrees]). EMG data were recorded from eight muscles contributing to the movements during the pre- and posttests. Most targets were acquired more rapidly after the practice period. Performance improvements were, in most target directions, accompanied by increases in the smoothness of the movement trajectories. When target acquisition required movement in both dfs, there were also practice-related decreases in the extent to which the trajectories deviated from a direct path to the target. The contribution of monofunctional muscles ( those producing torque in a single df) increased with practice during movements in which they acted as agonists. The activity in bifunctional muscles ( those contributing torque in both dfs) remained at pretest levels in most movements. The results suggest that performance gains were mediated primarily by changes in the spatial organization of muscles synergies. These changes were expressed most prominently in terms of the magnitude of activation of the monofunctional muscles.

Individual Palmitoyl Residues Serve Distinct Roles in H-ras Trafficking, Microlocalization, and Signaling

H-ras is anchored to the plasma membrane by two palmitoylated cysteine residues, Cys181 and Cys184, operating in concert with a C-terminal S-farnesyl cysteine carboxymethylester. Here we demonstrate that the two palmitates serve distinct biological roles. Monopalmitoylation of Cys181 is required and sufficient for efficient trafficking of H-ras to the plasma membrane, whereas monopallmitoylation of Cys184 does not permit efficient trafficking beyond the Golgi apparatus. However, once at the plasma membrane, monopalmitoylation of Cys184 supports correct GTP-regulated lateral segregation of H-ras between cbolesterol-dependent and cholesterol-independent microdomains. In contrast, monopallmitoylation of Cys181 dramatically reverses H-ras lateral segregation, driving GTP-loaded H-ras into cholesterol-dependent microdomains. Intriguingly, the Cys181 monopalmitoylated H-ras anchor emulates the GTP-regulated microdomain interactions of N-ras. These results identify N-ras as the Ras isoform that normally signals from lipid rafts but also reveal that spacing between palmitate and prenyl groups influences anchor interactions with the lipid bilayer. This concept is further supported by the different plasma membrane affinities of the monopalmitoylated anchors: Cys181-palmitate is equivalent to the dually palmitoylated wild-type anchor, whereas Cys184-pahnitate is weaker. Thus, membrane affinity of a pallmitoylated anchor is a function both of the hydrophobicity of the lipid moieties and their spatial organization. Finally we show that the plasma membrane affinity of monopahnitoylated anchors is absolutely dependent on cholesterol, identifying a new role for cholesterol in promoting interactions with the raft and nonraft plasma membrane.

Semantic caching for web-based spatial applications

Client-side caching of spatial data is an important yet very much under investigated issue. Effective caching of vector spatial data has the potential to greatly improve the performance of spatial applications in the Web and wireless environments. In this paper, we study the problem of semantic spatial caching, focusing on effective organization of spatial data and spatial query trimming to take advantage of cached data. Semantic caching for spatial data is a much more complex problem than semantic caching for aspatial data. Several novel ideas are proposed in this paper for spatial applications. A number of typical spatial application scenarios are used to generate spatial query sequences. An extensive experimental performance study is conducted based on these scenarios using real spatial data. We demonstrate a significant performance improvement using our ideas.