Protein phosphorylation by
kinases plays a central role in the regulation and coordination of multiple biological processes. In general, knowledge on
kinase specificity is restricted to substrates identified in the context of specific cellular responses, but
kinases are likely to have multiple additional substrates and be integrated in signaling networks that might be spatially and temporally different, and in which
protein complexes and subcellular localization can play an important role. In this report the substrate specificity of atypical human
vaccinia-related
kinases (VRK1 and VRK2) using a human
peptide-array containing 1080 sequences phosphorylated in known signaling pathways has been studied. The two
kinases identify a subset of potential
peptide targets, all of them result in a consensus sequence composed of at least four basic residues in
peptide targets. Linear
peptide arrays are therefore a useful approach in the characterization of
kinases and substrate identification, which can contribute to delineate the signaling network in which VRK
proteins participate. One of these target
proteins is coilin; a basic
protein located in nuclear Cajal bodies. Coilin is phosphorylated in Ser184 by both VRK1 and VRK2. Coilin colocalizes and interacts with VRK1 in Cajal bodies, but not with the mutant VRK1 (R358X). VRK1 (R358X) is less active than VRK1. Altered regulation of coilin might be implicated in several neurological diseases such as
ataxias and spinal
muscular atrophies.