A 'Build and Retrieve' methodology to simultaneously solve cryo-EM structures of membrane proteins.

Single-particle cryo-electron microscopy (cryo-EM) has become a powerful technique in the field of structural biology. However, the inability to reliably produce pure, homogeneous membrane protein samples hampers the progress of their structural determination. Here, we develop a bottom-up iterative method, Build and Retrieve (BaR), that enables the identification and determination of cryo-EM structures of a variety of inner and outer membrane proteins, including membrane protein complexes of different sizes and dimensions, from a heterogeneous, impure protein sample. We also use the BaR methodology to elucidate structural information from Escherichia coli K12 crude membrane and raw lysate. The findings demonstrate that it is possible to solve high-resolution structures of a number of relatively small (<100 kDa) and less abundant (<10%) unidentified membrane proteins within a single, heterogeneous sample. Importantly, these results highlight the potential of cryo-EM for systems structural proteomics. The iterative Build and Retrieve (BaR) methodology facilitates the solving of cryo-EM structures of multiple membrane (and soluble) proteins simultaneously, including small and low-abundance membrane proteins.