The overall goal of this proposal is to promote structure determination of integral membrane proteins by cryoelectron microscopy (cryo-EM) of 2D crystals. Membrane proteins are essential to all cells and favored drug targets, yet their 3D structures have proven difficult to ascertain. Cryo-EM has a demonstrated capability for structure determination at atomic resolution and the important advantage of maintaining integral membrane proteins within their native membrane environment. Nevertheless, the proliferation of high- throughput screening for 3D crystallization trials has given a distinct advantage to X-ray crystallography. We propose to implement analogous technologies for screening 2D crystallization trials of integral membrane proteins within lipid bilayers in order to reestablish cryo-EM as a viable alternative in the high-throughput, post-genomic era. We will form a partnership with the New York Consortium on Membrane Protein Structure (NYCOMPS), which is a Specialized Center of the NIH Protein Structure Initiative operating out of the New York Structural Biology Center. NYCOMPS will provide requested expression vectors from a large database of membrane proteins that they are screening for expression levels and homogeneity. After scaling up expression, we will use a robotic liquid handler to set up 2D crystallization trials by dialysis and prepare EM samples by negative stain in a 96-well format. Imaging currently represents a critical bottleneck for screening 2D crystal trials and we will implement technologies to automatically acquire images from these samples in the electron microscope. Thus, we expect to assess -50 different protein targets per year. The resulting information will be used to establish general principles governing the 2D crystallization process and, importantly, to produce 2D crystals that are suitable for structure determination at atomic resolution. Given a workable method to systematically search for and optimize 2D crystals, cryo-EM will become generally viable for high resolution structure determination and offer an important alternative to X-ray crystallography and NMR spectroscopy. In particular, the modest requirements for quantity and purity of proteins, as well as the natural environment provided by the lipid membrane, make cryo-EM an attractive alternative, especially as we move from bacterial proteomes towards large membrane protein complexes from eukaryotic cells that have proven more difficult to express and to maintain in a detergent solubilized state.