Protein engineering for selective proteomics.

Post-translational modifications, complex formation, subcellular localization, and cell-type-specific expression create functionally distinct protein subpopulations that enable living systems to execute rapid and precise responses to changing conditions. Systems-level analysis of these subproteomes remains challenging, requiring preservation of spatial information or enrichment of species that are transient and present at low abundance. Engineered proteins have emerged as important tools for selective proteomics based on their capacity for highly specific molecular recognition and their genetic targetability. Here, we focus on new developments in protein engineering for selective proteomics of post-translational modifications, protein complexes, subcellular compartments, and cell types. We also address remaining challenges and future opportunities to integrate engineered protein tools across different subproteome scales to map the proteome with unprecedented depth and detail.

Structure of an AMPK complex in an inactive, ATP-bound state.

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) regulates metabolism in response to the cellular energy states. Under energy stress, AMP stabilizes the active AMPK conformation, in which the kinase activation loop (AL) is protected from protein phosphatases, thus keeping the AL in its active, phosphorylated state. At low AMP:ATP (adenosine triphosphate) ratios, ATP inhibits AMPK by increasing AL dynamics and accessibility. We developed conformation-specific antibodies to trap ATP-bound AMPK in a fully inactive, dynamic state and determined its structure at 3.5-angstrom resolution using cryo-electron microscopy. A 180° rotation and 100-angstrom displacement of the kinase domain fully exposes the AL. On the basis of the structure and supporting biophysical data, we propose a multistep mechanism explaining how adenine nucleotides and pharmacological agonists modulate AMPK activity by altering AL phosphorylation and accessibility.

Synthetic antibodies against BRIL as universal fiducial marks for single-particle cryoEM structure determination of membrane proteins.

We propose the concept of universal fiducials based on a set of pre-made semi-synthetic antibodies (sABs) generated by customized phage display selections against the fusion protein BRIL, an engineered variant of apocytochrome b562a. These sABs can bind to BRIL fused either into the loops or termini of different GPCRs, ion channels, receptors and transporters without disrupting their structure. A crystal structure of BRIL in complex with an affinity-matured sAB (BAG2) that bound to all systems tested delineates the footprint of interaction. Negative stain and cryoEM data of several examples of BRIL-membrane protein chimera highlight the effectiveness of the sABs as universal fiducial marks. Taken together with a cryoEM structure of sAB bound human nicotinic acetylcholine receptor, this work demonstrates that these anti-BRIL sABs can greatly enhance the particle properties leading to improved cryoEM outcomes, especially for challenging membrane proteins.