A novel whole-cell lysate kinase assay identifies substrates of the p38 MAPK in differentiating myoblasts

Abstract

AbstractBackgroundThe p38α mitogen-activated protein kinase (MAPK) is a critical mediator of myoblast differentiation, and does so in part through the phosphorylation and regulation of several transcription factors and chromatin remodelling proteins. However, whether p38α is involved in processes other than gene regulation during myogenesis is currently unknown, and why other p38 isoforms cannot compensate for its loss is unclear.MethodsTo further characterise the involvement of p38α during myoblast differentiation, we developed and applied a simple technique for identifying relevantin vivokinase substrates and their phosphorylation sites. In addition to identifying substrates for one kinase, the technique can be usedin vitroto compare multiple kinases in the same experiment, and we made use of this to study the substrate specificities of the p38α and β isoforms.ResultsApplying the technique to p38α resulted in the identification of sevenin vivophosphorylation sites on six proteins, four of which are cytoplasmic, in lysate derived from differentiating myoblasts. Anin vitrocomparison with p38β revealed that substrate specificity does not discriminate these two isoforms, but rather that their distinguishing characteristic appears to be cellular localisation.ConclusionOur results suggest p38α has a novel cytoplasmic role during myogenesis and that its unique cellular localisation may be why p38β and other isoforms cannot compensate for its absence. The substrate-finding approach presented here also provides a necessary tool for studying the hundreds of protein kinases that exist and for uncovering the deeper mechanisms of phosphorylation-dependent cell signalling.