Waksman Institute and The Department of Molecular Biology and Biochemistry, Rutgers, The State University of New Jersey, 190 Frelinghuysen Rd., Piscataway 08854-8020, U.S.A.
A tobacco MAP kinase termed SIPK (salicylic acid-induced protein kinase) is activated in response to a variety of stress signals, including pathogen attack and wounding (S. Zhang and D. F. Klessig, Proc. Natl. Acad. Sci. USA 95:7225--7230, 1998; S. Zhang and D. F. Klessig, Proc. Natl. Acad. Sci. USA 95:7433--7438, 1998). Using the yeast two-hybrid system, we have identified a gene encoding a protein that interacts with SIPK but not the wounding induced protein kinase (WIPK), which is another tobacco MAP kinase. Sequence analysis indicated that this SIPK-interacting protein is a member of the MAP kinase kinase family; thus, it was named SIPK kinase (SIPKK). Co-immunoprecipitation experiments demonstrated that SIPKK and SIPK interact in vitro. Consistent with its putative function as a kinase, SIPKK phosphorylated myelin basic protein in vitro. Interestingly, SIPKK was induced at the mRNA level after Tobacco mosaic virus (TMV) infection or wounding, albeit with kinetics that are too slow to account for the activation of SIPK following these stimuli.