Reconsidering phosphorylation in the control of inducible CARD11 scaffold activity during antigen receptor signaling

Protein phosphorylation is a commonly used regulatory step that controls signal transduction pathways in a wide array of biological contexts. The finding that a residue is phosphorylated, coupled with the observation that mutation of that residue impacts signaling, often forms the basis for concluding that the phosphorylation of that residue is a key signaling step. However, in certain cases, the situation is more complicated and warrants further study to obtain a clear mechanistic understanding of whether and how the kinase-mediated modification in question is important. CARD11 is a multi-domain signaling scaffold that functions as a hub in lymphocytes to transmit the engagement of antigen receptors into the activation of NF-κB, JNK and mTOR. The phosphorylation of the CARD11 autoinhibitory Inhibitory Domain in response to antigen receptor triggering has been proposed to control the signal-induced conversion of CARD11 from an inactive to an active scaffold in a step required for lymphocyte activation. In this review, I discuss recent data that suggests that this model should be reconsidered for certain phosphorylation events in CARD11 and propose possible experimental avenues for resolution of raised issues.