14‐3‐3 proteins mediate inhibitory effects of cAMP on salt‐inducible kinases (SIKs)

The salt‐inducible kinase (SIK) family regulates cellular gene expression via the phosphorylation of cAMP‐regulated transcriptional coactivators (CRTCs) and class IIA histone deacetylases, which are sequestered in the cytoplasm by phosphorylation‐dependent 14‐3‐3 interactions. SIK activity toward these substrates is inhibited by increases in cAMP signaling, although the underlying mechanism is unclear. Here, we show that the protein kinase A (PKA)‐dependent phosphorylation of SIKs inhibits their catalytic activity by inducing 14‐3‐3 protein binding. SIK1 and SIK3 contain two functional PKA/14‐3‐3 sites, while SIK2 has four. In keeping with the dimeric nature of 14‐3‐3s, the presence of multiple binding sites within target proteins dramatically increases binding affinity. As a result, loss of a single 14‐3‐3‐binding site in SIK1 and SIK3 abolished 14‐3‐3 association and rendered them insensitive to cAMP. In contrast, mutation of three sites in SIK2 was necessary to fully block cAMP regulation. Superimposed on the effects of PKA phosphorylation and 14‐3‐3 association, an evolutionary conserved domain in SIK1 and SIK2 (the so called RK‐rich region; 595–624 in hSIK2) is also required for the inhibition of SIK2 activity. Collectively, these results point to a dual role for 14‐3‐3 proteins in repressing a family of Ser/Thr kinases as well as their substrates. Salt‐inducible kinases (SIKs) regulate gene expression by phosphorylating cAMP‐regulated transcriptional coactivators (CRTCs) and class IIA histone deacetylases (HDACs), promoting their subsequent interaction with 14‐3‐3 proteins and cytoplasmic sequestration. Although cAMP signalling is known to inhibit SIK activity towards its substrates, the underlying mechanism was unclear. Marc Montminy and colleagues now report that protein kinase A‐mediated phosphorylation inactivates SIKs by inducing 14‐3‐3 protein binding. Their work reveals an interesting role for 14‐3‐3 proteins in inhibiting SIKs and their substrates.