Molecular Model of the Microvillar Cytoskeleton and Organization of the Brush Border

BACKGROUND. Brush border microvilli are ~1-µm long finger-like projections emanating from the apical surfaces of certain, specialized absorptive epithelial cells. A highly symmetric hexagonal array of thousands of these uniformly sized structures form the brush border, which in addition to aiding in nutrient absorption also defends the large surface area against pathogens. Here, we present a molecular model of the protein cytoskeleton responsible for this dramatic cellular morphology. METHODOLOGY/PRINCIPAL FINDINGS. The model is constructed from published crystallographic and microscopic structures reported by several groups over the last 30+ years. Our efforts resulted in a single, unique, self-consistent arrangement of actin, fimbrin, villin, brush border myosin (Myo1A), calmodulin, and brush border spectrin. The central actin core bundle that supports the microvillus is nearly saturated with fimbrin and villin cross-linkers and has a density similar to that found in protein crystals. The proposed model accounts for all major proteinaceous components, reproduces the experimentally determined stoichiometry, and is consistent with the size and morphology of the biological brush border membrane. CONCLUSIONS/SIGNIFICANCE. The model presented here will serve as a structural framework to explain many of the dynamic cellular processes occurring over several time scales, such as protein diffusion, association, and turnover, lipid raft sorting, membrane deformation, cytoskeletal-membrane interactions, and even effacement of the brush border by invading pathogens. In addition, this model provides a structural basis for evaluating the equilibrium processes that result in the uniform size and structure of the highly dynamic microvilli.Boston University (Graduate Student Research Fellowship); National Institutes of Health (GM62886

The Asypow S(plus) Library for Asymptotic Power Calculations

The asypow library consists of routines written in the S language that calculate power and related quantities utilizing asymptotic methods. A paper describing these methods with examples is in preparation [1]. Two methods are available. The likelihood ratio method (LR) is described in [2]. Another general method appears recently in [3]; and we designate it the SMO method after the initials of the authors.

Type I X-ray Bursts at Low Accretion Rates

Neutron stars, with their strong surface gravity, have interestingly short timescales for the sedimentation of heavy elements. Recent observations of unstable thermonuclear burning (observed as X-ray bursts) on the surfaces of slowly accreting neutron stars ($< 0.01$ of the Eddington rate) motivate us to examine how sedimentation of CNO isotopes affects the ignition of these bursts. We further estimate the burst development using a simple one-zone model with a full reaction network. We report a region of mass accretion rates for weak H flashes. Such flashes can lead to a large reservoir of He, the unstable burning of which may explain some observed long bursts (duration $\sim 1000$ s).Comment: 6 pages, 2 figures, submitted to the proceedings of the conference "The Multicoloured Landscape of Compact Objects and Their Explosive Origins'', 2006 June 11--24, Cefalu, Sicily (Italy), to be published by AI

Alternatives to Charcoal for Improving Chronometric Dating of Puget Sound Archaeological Sites

Radiocarbon dating of archaeological sites in the Puget Lowlands can be problematic. Dating specific cultural events associated with features and sites is difficult due to the ubiquity of charcoal in forest soils and poor preservation of bone in acidic soils. These conditions have impeded the development of regional cultural chronologies. The lack of dates for critical time periods also inhibits testing processual models of cultural change. Evidence for the timing and rate of ecological, economic, and political change is critical for testing evolutionary models in the Pacific Northwest (PNW). Radiocarbon dating highly burned bone (calcined bone) and luminescence dating fire-modified rock from cooking features will improve age estimates for features and sites. Calcined bone survives well in archaeological sites with acidic soils that are common in the PNW. Luminescence dating can be applied to fire-modified rock recovered particularly from food processing features. This study, conducted in collaboration with the DirectAMS and the University of Washington Luminescence Laboratory, summarizes tests designed to compare dates for paired samples of charcoal, calcined bone, and fire-modified rock. The comparisons are based on a model that includes both the nature of target events and properties of the dated material. Test results show the accuracy and precision of radiocarbon dates for calcined bone and substantiate the utility of luminescence dates. As possible, two or more of the dating methods should be used together to assign age estimates for features and sites. Within the next 20 years, we may have accumulated sufficient chronometric dates to better outline cultural chronologies for the Puget Sound. More complete chronometric databases and cultural outlines will then better support tests of processual models of cultural changes in the Pacific Northwest

Forest diagrams for elements of Thompson's group F

We introduce forest diagrams to represent elements of Thompson's group F. These diagrams relate to a certain action of F on the real line in the same way that tree diagrams relate to the standard action of F on the unit interval. Using forest diagrams, we give a conceptually simple length formula for elements of F with respect to the {x_0,x_1} generating set, and we discuss the construction of minimum-length words for positive elements. Finally, we use forest diagrams and the length formula to examine the structure of the Cayley graph of F.Comment: 44 pages, 70 figure

JPL reuse program

The goal of the JPL reuse activity is to develop a quantitative understanding of the factors which encourage or inhibit software reuse, and of productivity improvements achievable through reuse. The primary activity is the measurement of parameters relevant to reuse in the environment of actual projects. The program has three objectives: (1) to develop a model to allow assessment of competing reuse techniques, (2) to extend reuse from the unit to the sub-system level, and (3) to expand from specific applications to a broader application domain. Application domains, which apply to all interplanetary projects, include Mission Operations, Science Information Systems, Flight Software, and Simulations. The program is targeting all phases and activities of the life cycle and a full range of software products. The approach will be both experimental (observe, hypothesize and evaluate) and constructive (introduce new tools and techniques). The primary target projects are Deep Space Network activities - the Ground Facilities facility upgrade, the Network Operations Control Center upgrade, and the Signal Processing Center. This is the first group of closely related projects being done in Ada at JPL. A reuse base will be developed initially by classifying potentially reusable components from one project; it will be used and expanded with additional projects

The Asypow S(plus) Library for Asymptotic Power Calculations

The asypow library consists of routines written in the S language that calculate power and related quantities utilizing asymptotic methods. A paper describing these methods with examples is in preparation [1]. Two methods are available. The likelihood ratio method (LR) is described in [2]. Another general method appears recently in [3]; and we designate it the SMO method after the initials of the authors