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.