ERK5 kinase activity is dispensable for cellular immune response and proliferation

Whole protein deletion and pharmacological inhibition are frequently used to functionally annotate enzymes. Each has limitations: whole protein deletion removes both enzymatic and nonenzymatic functions, and small molecule inhibitors can have unrecognized off-target activities. When both approaches agree, it's nearly incontrovertible support for protein function. Here we describe a counterexample. ERK5 knockdown and inhibition supported a role for this kinase in a number of biological processes. We show that previously reported ERK5 compounds inhibit bromodomain-containing proteins (BRDs) sufficiently to account for their phenotypic effects. We describe highly specific inhibitors of ERK5 that do not inhibit BRDs. With these, we show that cellular inflammation and proliferation are not dependent on ERK5 catalytic activity, thus making ERK5 unique among the MAP kinases.