Systematic analysis of membrane contact sites in Saccharomyces cerevisiae uncovers modulators of cellular lipid distribution

Actively maintained close appositions, or contact sites, between organelle membranes, enable the efficient transfer of biomolecules between the various cellular compartments. Several such sites have been described together with their tethering machinery. Despite these advances we are still far from a comprehensive understanding of the function and regulation of most contact sites. To systematically characterize the proteome of contact sites and support the discovery of new tethers and functional molecules, we established a high throughput screening approach in Saccharomyces cerevisiae based on co-localization imaging. We imaged split fluorescence reporters for six different contact sites, two of which have never been studied before, on the background of 1165 strains expressing a mCherry-tagged yeast protein that have a cellular punctate distribution (a hallmark of contact sites). By scoring both co-localization events and effects on reporter size and abundance, we discovered over 100 new potential contact site residents and effectors in yeast. Focusing on several of the newly identified residents, we identified one set of hits as previously unrecognized homologs to Vps13 and Atg2. These proteins share their lipid transport domain, thus expanding this family of lipid transporters. Analysis of another candidate, Ypr097w, which we now call Lec1 (Lipid-droplet Ergosterol Cortex 1), revealed that this previously uncharacterized protein dynamically shifts between lipid droplets and the cell cortex, and plays a role in regulation of ergosterol distribution in the cell

Systematic analysis of membrane contact sites in Saccharomyces cerevisiae uncovers modulators of cellular lipid distribution

Actively maintained close appositions between organelle membranes, also known as contact sites, enable the efficient transfer of biomolecules between cellular compartments. Several such sites have been described as well as their tethering machineries. Despite these advances we are still far from a comprehensive understanding of the function and regulation of most contact sites. To systematically characterize contact site proteomes, we established a high-throughput screening approach in Saccharomyces cerevisiae based on co-localization imaging. We imaged split fluorescence reporters for six different contact sites, several of which are poorly characterized, on the background of 1165 strains expressing a mCherry-tagged yeast protein that has a cellular punctate distribution (a hallmark of contact sites), under regulation of the strong TEF2 promoter. By scoring both co-localization events and effects on reporter size and abundance, we discovered over 100 new potential contact site residents and effectors in yeast. Focusing on several of the newly identified residents, we identified three homologs of Vps13 and Atg2 that are residents of multiple contact sites. These proteins share their lipid transport domain, thus expanding this family of lipid transporters. Analysis of another candidate, Ypr097w, which we now call Lec1 (Lipid-droplet Ergosterol Cortex 1), revealed that this previously uncharacterized protein dynamically shifts between lipid droplets and the cell cortex, and plays a role in regulation of ergosterol distribution in the cell. Overall, our analysis expands the universe of contact site residents and effectors and creates a rich database to mine for new functions, tethers, and regulators

Flame speed predictions in planar micro/mesoscale combustors with conjugate heat transfer

An analytical model for flame stabilization in meso-scale channels is developed by solving the two-dimensional partial differential equations associated with heat transport in the gas and structure and species transport in the gas. It improves on previous models by eliminating the need to assume values for the Nusselt numbers in the pre and post-flame regions. The effects of heat loss to the environment, wall thermal conductivity, and wall geometry on the burning velocity and extinction are explored. Extinction limits and fast and slow burning modes are identified but their dependence on structure thermal conductivity and heat losses differ from previous quasi one-dimensional analyses. Heat recirculation from the post-flame to the pre-flame is shown to be the primary mechanism for flame stabilization and burning rate enhancement in micro-channels. Combustor design parameters like the wall thickness ratio, thermal conductivity ratio, and heat loss to the environment each influence the flame speed through their influence on the total heat recirculation. These findings are used to propose a simple methodology for preliminary micro-combustor design

2D analytical model investigating the effect of structural heat recirculation on the temperature profiles in a parallel plate reactor with a premixed laminar flame

A two-dimensional model for heat transfer in reacting channel flow is developed along with an analytical solution that relates the temperature field in the channel to the flow Pe number. The solution is derived from first principles by modeling the flame as a volumetric heat source and by applying “jump conditions” across the flame. The model explores the role of heat recirculation via the channel's walls by accounting for the thermal coupling between the wall and the gas. The uniqueness of the model lies in that it is developed by simultaneously solving the two dimensional temperature fields in both the wall and structure analytically. The solution is obtained using separation of variables in the streamwise (x) and the transverse (y) direction. Thermal coupling between the wall and gas is achieved by requiring that the temperature and heat flux match at the interface. The outer wall boundary can be either adiabatic or have a convective heat loss based on Newton's law of cooling. The resulting solution is a Fourier series (for both wall and gas temperature fields) which depends on the flow Pe and the outer wall boundary condition. This simple model and the resulting analytical solution provide an extremely computationally efficient tool for exploring the effects of varying channel height and gas velocity on the temperature distribution associated with reacting (combusting) flow a channel. Understanding these tradeoffs is important for developing miniaturized, combustion-based power sources

Modeling and simulation of fuel-oxidizer mixing in micropower systems

This paper estimates the range of Reynolds numbers and diffusive mixing lengths associated with fuel-oxidizer mixing in micropower systems and then develops analytical and numerical models to explore how mixing performance varies with device size. Both axial and transverse diffusion of species are considered. The models show that Reynolds numbers associated with mixing in micropower systems fall in the laminar-transitional flow regime, where relatively little experimental data exists. They also indicate that fuel-oxidizer mixing lengths decrease with decreasing device size and that the relative importance of axial diffusion to fuel-oxidizer mixing on the microscale depends on the ratio of the diffusive to the convective velocity. At high flow velocities, the mixing length is proportional to the convective velocity and the physical dimensions of the device. At low flow velocities, diffusion dominates, and the mixing length is only proportional to the physical dimensions of the device. This transition in behavior is the result of axial diffusion, which becomes important at Re < 20. Overall, these results suggest that axial diffusion may impose additional limits on the degree to which a combustion-based micropower system can be miniaturized. Copyrigh

SCALING OF FUEL CELL SYSTEMS FOR MICRO-POWER APPLICATIONS

Abstract: A model for miniature direct methanol fuel cell systems was constructed and used to investigate the scaling of small fuel cell system performance. The model accounts for the scaling of component performance in order to properly represent the balance of plant. The model results indicate that a 120 gram direct methanol fuel cell system can achieve power densities in excess of 0.1 W/g and maintain this power level for several hours. Power density can be improved by decreasing the number of cells and the fuel mass fraction at the expense of reduced endurance. While this power density is approximately one order of magnitude smaller than what can be attained using a battery, the fuel cell system has much higher energy density and thus can operate for much longer periods

Integration of catalytic combustion and heat recovery with meso-scale solid oxide fuel cell system

To facilitate high-power density operation of a meso-scale solid oxide fuel cell (SOFC) system, fuel processing and anode exhaust catalytic combustor with waste heat recovery are critical components. An integrated modeling study of a catalytic combustor with a solid oxide fuel cell and a catalytic partial oxidation (CPOx) reactor indicates critical aspects of the butane-fueled system design in order to ensure stable operation of the SOFC as well as the combustor and CPOx reactor. The modeled system consists of: 1) a Rh-coated ceramic foam catalytic partial oxidation reactor, 2) a SOFC with a Ni/YSZ structural anode, a dense YSZ electrolyte, and a LSM/YSZ cathode layer, and 3) a Pt-coated anode exhaust combustor with waste heat recovery. Model results for a system designed to produce < 30 W electric power from n-butane show how the design of the inlet-air cooled catalytic combustor can maximize combustion efficiency of the anode exhaust and heat recovery to the system inlet air flow. The model also shows the need to minimize heat loss in the air flow passages in order to maintain stable SOFC operation at 700 °C or higher. There is a strong sensitivity of the system operation to the SOFC operating voltage as well as the overall air to fuel ratio, and these sensitivities place important bounds on the range of operating conditions

Heat transfer in mini/micro channels with combustion: A simple analysis for application in non-intrusive IR diagnostics

An analytical solution for the temperature distribution in 2D laminar reacting flow between closely spaced parallel plates is derived as part of a larger effort to develop a nonintrusive technique for measuring gas temperature distributions in millimeter and submillimeter scale channel flows. The results show that the exact solution, a Fourier series, which is a function of the Peclet number, is approximated by second and fourth order polynomial fits to an R value of almost unity for both fits. The slopes of the temperature near the wall (heat fluxes) are captured to within 20% of the exact solution using a second order polynomial and to within 2% of the exact solution using a fourth order polynomial. The fits are used in a nonintrusive Fourier transform infrared spectroscopy technique and enable one to infer the temperature distribution along an absorbing gas column from the measured absorption spectrum. The technique is demonstrated in a silicon-walled microcombustor