A chemical genetics approach to examine the functions of AAA proteins

Allosteric activation of VCP, an AAA unfoldase, by small molecule mimicry

The loss of function of AAA (ATPases associated with diverse cellular activities) mechanoenzymes has been linked to diseases, and small molecules that activate these proteins can be powerful tools to probe mechanisms and test therapeutic hypotheses. Unlike chemical inhibitors that can bind a single conformational state to block enzyme function, activator binding must be permissive to different conformational states needed for mechanochemistry. However, we do not know how AAA proteins can be activated by small molecules. Here, we focus on valosin-containing protein (VCP)/p97, an AAA unfoldase whose loss of function has been linked to protein aggregation-based disorders, to identify druggable sites for chemical activators. We identified VCP ATPase Activator 1 (VAA1), a compound that dose-dependently stimulates VCP ATPase activity up to ~threefold. Our cryo-EM studies resulted in structures (ranging from ~2.9 to 3.7 Å-resolution) of VCP in apo and ADP-bound states and revealed that VAA1 binds an allosteric pocket near the C-terminus in both states. Engineered mutations in the VAA1-binding site confer resistance to VAA1, and furthermore, modulate VCP activity. Mutation of a phenylalanine residue in the VCP C-terminal tail that can occupy the VAA1 binding site also stimulates ATPase activity, suggesting that VAA1 acts by mimicking this interaction. Together, our findings uncover a druggable allosteric site and a mechanism of enzyme regulation that can be tuned through small molecule mimicry.

Allosteric activation of VCP, a AAA unfoldase, by small molecule mimicry

The loss of function of AAA (ATPases associated with diverse cellular activities) mechanoenzymes has been linked to diseases, and small molecules that activate these proteins can be powerful tools to probe mechanisms and test therapeutic hypotheses. Unlike chemical inhibitors that can bind a single conformational state to block enzyme activity, activator binding must be permissive to different conformational states needed for enzyme function. However, we do not know how AAA proteins can be activated by small molecules. Here, we focus on valosin-containing protein (VCP)/p97, a AAA unfoldase whose loss of function has been linked to protein aggregation-based disorders, to identify druggable sites for chemical activators. We identified VCP Activator 1 (VA1), a compound that dose-dependently stimulates VCP ATPase activity up to ∼3-fold. Our cryo-EM studies resulted in structures (∼2.9-3.5 Å-resolution) of VCP in apo and ADP-bound states, and revealed VA1 binding an allosteric pocket near the C-terminus in both states. Engineered mutations in the VA1 binding site confer resistance to VA1, and furthermore, modulate VCP activity to a similar level as VA1-mediated activation. The VA1 binding site can alternatively be occupied by a phenylalanine residue in the VCP C-terminal tail, a motif that is post-translationally modified and interacts with cofactors. Together, our findings uncover a druggable allosteric site and a mechanism of enzyme regulation that can be tuned through small molecule mimicry.

Significance

The loss of function of valosin-containing protein (VCP/p97), a mechanoenzyme from the AAA superfamily that hydrolyzes ATP and uses the released energy to extract or unfold substrate proteins, is linked to protein aggregation-based disorders. However, druggable allosteric sites to activate VCP, or any AAA mechanoenzyme, have not been identified. Here, we report cryo-EM structures of VCP in two states in complex with VA1, a compound we identified that dose-dependently stimulates VCP's ATP hydrolysis activity. The VA1 binding site can also be occupied by a phenylalanine residue in the VCP C-terminal tail, suggesting that VA1 acts through mimicry of this interaction. Our study reveals a druggable allosteric site and a mechanism of enzyme regulation.

Achieving the promise and avoiding the peril of chemical probes using genetics

Chemical probes can be valuable tools for studying protein targets, but addressing concerns about a probe's cellular target or its specificity can be challenging. A reliable strategy is to use a mutation that does not alter a target's function but confers resistance (or sensitizes) to the inhibitor in both cellular and biochemical assays. However, challenges remain in finding such mutations. Here, we discuss structure- and cell-based approaches to identify resistance- and sensitivity-conferring mutations. Further, we describe how resistance-conferring mutations can help with compound design, and the use of saturation mutagenesis to characterize a compound binding site. We highlight how genetic approaches can ensure the proper use of chemical inhibitors to pursue mechanistic studies and test therapeutic hypotheses.

The AAA+ superfamily: a review of the structural and mechanistic principles of these molecular machines

ATPases associated with diverse cellular activities (AAA+ proteins) are a superfamily of proteins found throughout all domains of life. The hallmark of this family is a conserved AAA+ domain responsible for a diverse range of cellular activities. Typically, AAA+ proteins transduce chemical energy from the hydrolysis of ATP into mechanical energy through conformational change, which can drive a variety of biological processes. AAA+ proteins operate in a variety of cellular contexts with diverse functions including disassembly of SNARE proteins, protein quality control, DNA replication, ribosome assembly, and viral replication. This breadth of function illustrates both the importance of AAA+ proteins in health and disease and emphasizes the importance of understanding conserved mechanisms of chemo-mechanical energy transduction. This review is divided into three major portions. First, the core AAA+ fold is presented. Next, the seven different clades of AAA+ proteins and structural details and reclassification pertaining to proteins in each clade are described. Finally, two well-known AAA+ proteins, NSF and its close relative p97, are reviewed in detail.