Molecular cloning and characterization of maize ZmMEK1, a protein kinase with a catalytic domain homologous to mitogen- and stress-activated protein kinase kinases

Mitogen- or stress-activated protein kinase kinases (M/SAPKKs) are dual-specificity protein kinases that are components of highly conserved signal transduction pathways. A cDNA clone (ZmMEK1) was isolated from a Zea mays (L.) root tip library. ZmMEK1 contains a complete open reading frame, encoding a 355-amino-acid protein with an estimated molecular mass of 39,874 Da. The predicted protein contains the 11 catalytic sub-domains conserved in all protein kinases, and a version of the sub-domain VIII S/TxxxS/TxVCJT motif that is characteristic of M/SAPKK proteins (EC 2.7.1.37). The catalytic domain of ZmMEK1 is most closely related (65% identity) to the tomato M/SAPKK homolog LeMEK1, but exhibits similar identity (39-60%) to M/SAPKKs from other plants, animals and fungi. Northern blotting revealed ZmMEK1 mRNA in maize seedling roots and coleoptiles; in mature plants transcripts were detected in stems and low levels in leaves. Transcription of ZmMEK1 mRNA was also affected by environmental stimuli. The catalytic domain of ZmMEK1 was expressed as a glutathione-S-transferase (GST) fusion protein in Escherichia coli. Nanogram quantities of the purified fusion protein reacted with anti-M/SAPKK antibodies on immunoblots. In vitro, the GST-ZmMEK1 fusion protein undergoes autophosphorylation, and will phosphorylate myelin basic protein, but will not phosphorylate histone H1. ZmMEK1 encodes an enzyme that is structurally and functionally similar to other M/SAPKK proteins.