Membrane proteins constitute ~30% of prokaryotic and eukaryotic genomes but comprise a small fraction of the entries in protein structural databases. A number of features of membrane proteins render them challenging targets for the structural biologist, among which the most important is the difficulty in obtaining sufficient quantities of purified protein. We are exploring procedures to express and purify large numbers of prokaryotic membrane proteins. A set of 280 membrane proteins from Escherichia coli and Thermotoga maritima, a thermophile, was cloned and tested for expression in Escherichia coli. Under a set of standard conditions, expression could be detected in the membrane fraction for approximately 30% of the cloned targets. About 22 of the highest expressing membrane proteins were purified, typically in just two chromatographic steps. There was a clear correlation between the number of predicted transmembrane domains in a given target and its propensity to express and purify. Accordingly, the vast majority of successfully expressed and purified proteins had six or fewer transmembrane domains. We did not observe any clear advantage to the use of thermophilic targets. Two of the purified membrane proteins formed crystals. By comparison with protein production efforts for soluble proteins, where approximately 70% of cloned targets express and approximately 25% can be readily purified for structural studies [Christendat et al. (2000) Nat. Struct. Biol., 7, 903], our results demonstrate that a similar approach will succeed for membrane proteins, albeit with an expected higher attrition rate.

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A novel solution to integrating offshore wind power via the Fractional Frequency Transmission System (FFTS) is introduced in this paper. The basic idea of FFTS is to improve the technical and economic performance of AC transmission systems by lowering the grid frequency. FFTS is especially suitable for transmitting offshore wind power because the low frequency dramatically reduces the charging current in the cable. In addition, as a novel AC transmission system, FFTS performs much better than HVDC in constructing a multi-terminal (MT) grid. The basic structures and characteristics of a typical PMSG-based FFTS offshore wind power system and a MT-FFTS offshore grid are described first. Then two offshore wind power cases are used to compare the technical and economical pros and cons of the FFTS solution with the conventional solutions: HVAC and HVDC. The feasibility studies show that the performance of FFTS offshore wind power system is superior to that of HVAC and HVDC in both point-to-point (PP) and multi-terminal cases. The uniform annual value of PP-FFTS and MT-FFTS are 5.01% and 5.07% cheaper than PP-HVDC and MT-HVDC. Therefore, FFTS is a promising solution to offshore wind power integration and building offshore grids.Index Terms-Feasibility study; fractional frequency transmission system; offshore grid; offshore wind power integration

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