Identification of candidate substrates of SIK2 in vitro

Abstract

SIK2 is a serine/threonine kinase widely expressed in rat retina and it is postulated to be a potential regulator of FGF signal transduction in retina. The aim of this work is to explore a link between SIK2 and FGF/ERK pathway. In this context, three novel candidate SIK2 substrates, Gab1, Grb2 and ARaf that involved in this pathway and have consensus SIK2 phosphorylation motifs were identified by bioinformatics tools. Even though the search found no canonical SIK2 phosphorylation motif on Frs2 and Shc1 proteins, these were included in this study, because of their proposed regulatory roles in FGF signal transduction pathway and by having numerous potentially phosphorylatable serine and threonine residues. Subsequently, these candidates were expressed in bacteria as GST fusion proteins, purified through affinity columns and their phosphorylation by SIK2 was assayed using radioactively labeled ATP in vitro. We have shown two components of FGF signal pathway, Gab1 and A-Raf, that can be phosphorylated by SIK2 in vitro. These proteins are proposed to have major roles in FGF/ERK pathway, determining the level and duration of ERK activity. The duration of ERK activity (transient or sustained) suggested to be a determining factor in proliferation or differentiation responses in some cell types. It is conceivable that the phosphorylation status of these proteins defined by SIK2 may fine-tune the levels and duration of ERK activity, thus SIK2 may be a component of the events leading to proliferation versus differentiation decisions upon growth factor stimulation.