PSKH1 kinase activity is differentially modulated via allosteric binding of Ca²⁺ sensor proteins

Protein Serine Kinase H1 (PSKH1) was recently identified as a crucial factor in kidney development and is overexpressed in prostate, lung, and kidney cancers. However, little is known about PSKH1 regulatory mechanisms, leading to its classification as a "dark" kinase. Here, we used biochemistry and mass spectrometry to define PSKH1's consensus substrate motif, protein interactors, and how interactors, including Ca²⁺ sensor proteins, promote or suppress activity. Intriguingly, despite the absence of a canonical Calmodulin binding motif, Ca²⁺-Calmodulin activated PSKH1 while, in contrast, the ER-resident Ca²⁺ sensor of the Cab45, Reticulocalbin, Erc55, Calumenin (CREC) family, Reticulocalbin-3, suppressed PSKH1 catalytic activity. In addition to antagonistic regulation of the PSKH1 kinase domain by Ca²⁺ sensing proteins, we identified UNC119B as a protein interactor that activates PSKH1 via direct engagement of the kinase domain. Our findings identify complementary allosteric mechanisms by which regulatory proteins tune PSKH1's catalytic activity and raise the possibility that different Ca²⁺ sensors may act more broadly to tune kinase activities by detecting and decoding extremes of intracellular Ca²⁺ concentrations.