Arabidopsis sucrose non-fermenting-1-related protein kinase-1 and calcium-dependent protein kinase phosphorylate conserved target sites in ABA response element binding proteins

Evidence is provided that plant transcription factors of the ABA response element binding protein (AREBP) class are phosphorylated by sucrose non-fermenting-1-related protein kinase-1 (SnRK1) and a calcium-dependent protein kinase at highly conserved target sites. Two target sites for SnRK1 were identified in AREBPs using a specially developed motif search pipeline. Peptides containing the AREBP sites were phosphorylated by purified SnRK1 in vitro and by calcium-dependent and calcium-independent activities present in soluble protein extracted from Arabidopsis seedlings grown in liquid culture. Most (69-77%) of the calcium-independent phosphorylation of these peptides was removed by immunoprecipitation using anti-SnRK1 antisera, linking this activity unambiguously to SnRK1. It is possible that the protein kinase responsible for the calcium-dependent activity was SnRK3, a protein kinase that is related to SnRK1 and which has similar requirements for target site recognition. However, the involvement of other calcium-dependent protein kinases cannot be ruled out.