Activity and autophosphorylation of LAMMER protein kinases

Clk/STY, the murine homologue of the recently described LAMMER family of protein kinases, autophosphorylates on serine/threonine and tyrosine residues in vitro and in vivo. LAMMER kinases are found throughout eukaryotes and possess virtually complete amino acid identity in many domains critical for kinase function, leading to the question of whether other family members also possess dual specificity. We report here that the Drosophila family member DOA, human SK-G1, and the Saccharomyces cerevisiae KNS1, all possess protein kinase activity and autophosphorylate with dual specificity in vitro, suggesting that the entire family possesses this activity. Although the LAMMER kinases are closely related to the mitogen-activated protein kinase family, they possess different substrate specificity in vitro, based on phosphorylation of peptide and protein substrates and sequencing of a phosphorylation site in a common substrate.