Mouse Erk-1 gene product is a serine/threonine protein kinase that has the potential to phosphorylate tyrosine.

Bacterial expression of mouse gene Erk-1 yielded an active kinase with the same substrate specificity shown for ERK1 protein purified from rat cells. Although rat gene ERK1 is believed to encode a serine/threonine kinase based on sequence data and known ERK1 substrate phosphorylation sites, bacterially-produced mouse Erk-1 (bt-Erk-1) autophosphorylated on tyrosine in addition to serine and threonine residues. The bt-Erk-1 protein also had the capacity to reactivate the ribosomal protein S6 kinase (S6KII). Furthermore, treatment of bt-Erk-1 with either serine/threonine-specific phosphatase 2A or tyrosine-specific phosphatase 1B significantly decreased its kinase activity. These findings predict that autophosphorylation may play an important role in Erk-1/ERK1 regulation.