Topographs of two-dimensional porin OmpF crystals reconstituted in the presence of lipids were recorded in solution by atomic force microscopy (AFM) to a lateral resolution of 10 angstroms and a vertical resolution of 1 angstrom. Protein-protein interactions were demonstrated on the basis of the AFM results and earlier crystallographic findings. To assess protein-lipid interactions, the bilayer was modeled with kinked lipids by fitting the head groups to contours determined with AFM. Finally, two conformations of the extracellular porin surface were detected at forces of 0.1 nanonewton, demonstrating the potential of AFM to monitor conformational changes with high resolution.

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This Working Paper should not be reported as representing the views of the IMF.The views expressed in this Working Paper are those of the author(s) and do not necessarily represent those of the IMF or IMF policy. Working Papers describe research in progress by the author(s) and are published to elicit comments and to further debate.The literature measuring the impact of Preferential Trade Agreements (PTA) and WTO membership on trade flows has produced remarkably diverse results. Rose's (2004) seminal paper reports a range of specifications that show no WTO effects, but Subramanian and Wei (2007) contend that he does not fully control for multilateral resistance (which could bias WTO estimates). Subramanian and Wei (2007) address multilateral resistance comprehensively to report strong WTO trade effects for industrialized countries but do not account for unobserved bilateral heterogeneity (which could inflate WTO estimates). We unify these two approaches by accounting for both multilateral resistance and unobserved bilateral heterogeneity, while also allowing for individual trade effects of PTAs. WTO effects vanish and remain insignificant throughout once multilateral resistance, unobserved bilateral heterogeneity, and individual PTA effects are introduced. The result is robust to the use of alternative definitions and coding conventions for WTO membership that have been employed by Rose (2004), Tomz et al. (2007, or by Subramanian and Wei's (2007).

This paper develops new, far more extensive estimates of export quality, covering 178 countries and hundreds of products over 1962-2010. Quality upgrading is particularly rapid during the early stages of development, with quality convergence largely completed as a country reaches upper middle-income status. There is significant cross-country heterogeneity in quality growth rates. Within any given product line, quality converges both conditionally and unconditionally to the world frontier; increases in institutional quality and human capital are associated with faster quality upgrading. In turn, faster growth in quality is associated with more rapid output growth. The evidence suggests that quality upgrading is best encouraged through a broadly conducive domestic environment, rather than sector-specific policies. Diversification is important to create new upgrading opportunities.

The effect of preferential trade agreements (PTAs) on trade flows is subject to model uncertainty stemming from the diverse and even contradictory effects suggested by the theoretical PTA literature. The existing empirical literature has produced remarkably disparate results and the wide variety of empirical approaches reflects the uncertainty about the 'correct' set of explanatory variables that ought to be included in the analysis. To account for the model uncertainty that surrounds the validity of the competing PTA theories, we introduce Bayesian model averaging (BMA) to the PTA literature. Statistical theory shows that BMA successfully incorporates model uncertainty in linear regression analysis by minimizing the mean squared error, and by generating predictive distributions with optimal predictive performance. Once model uncertainty is addressed as part of the empirical strategy, we find strong evidence of trade creation, trade diversion, and open bloc effects. Our results are robust to a range of alternative empirical specifications proposed by the recent PTA literature.In this paper we apply Bayesian model averaging (BMA) to the PTA literature to re-examine model uncertainty. BMA is specifically designed to incorporate model uncertainty into the estimation process and is firmly rooted in statistical theory. It is a methodology that explores the model space without restrictions, weighs each model according to quality, and provides a probability distribution for each coefficient estimate. Raftery and Zheng (2003) show that BMA maximizes predictive performance while minimizing the total error rate when compared to any individual model. The rapidly growing list of economics applications using BMA include policy evaluations (e.g. Brock et al., 2003), monetary policy (e.g. Levin and Williams, 2003), macroeconomic forecasting (e.g. Garratt et al., 2003), economic growth (e.g. Fernandez et al., 2001), and international economics (e.g. Chen and Rogoff, 2006). The issue of model uncertainty surrounding PTA effects is well known in the PTA literature. Seldom do papers present less than a dozen different PTA regression specifications. We show that BMA overturns the fundamental Ghosh and Yamarik result by identifying a number of PTAs that exert decisive effects on trade flows. Since Ghosh and Yamarik, the PTA literature has evolved to introduce a number of innovations that address omitted variable bias. We show that our main finding of measurable PTA effects on trade flows is robust, even when the Ghosh and Yamarik (2004) dataset is updated to include additional years, additional PTAs, and alternative fixed-effect specifications. 1 Our methodological extensions include a full account of multilateral resistance (see, for example, Anderson and van Wincoop,