Cloning and characterization of MST4, a novel Ste20-like kinase

MST4, a novel member of the germinal center kinase subfamily of human Ste20 -like kinases, was cloned and characterized. Composed of a C-terminal regul atory domain and an N-terminal kinase domain, MST4 is most closely related to mammalian Ste20 kinase family member MST3. Both the kinase and C-termina l regulatory domains of MST4 are required for full activation of the kinase . Northern blot analysis indicates that MST4 is ubiquitously distributed, a nd the MST4 gene is localized to chromosome Xq26, a disease-rich region, by fluorescence in situ hybridization. Although some members of the MST4 fami ly function as upstream regulators of mitogen-activated protein kinase casc ades, expression of MST4 in 293 cells was not sufficient to activate or pot entiate extracellular signal-regulated kinase, c-Jun N-terminal kinase, or p38 kinase. An alternatively spliced isoform of MST4 (MST4a) was isolated b y yeast two-hybrid interaction with the catalytic domain of Raf from a huma n fetal brain cDNA library and also found in a variety of human fetal and a dult tissues. MST4a lacks an exon encoding kinase subdomains M-XI that stab ilizes substrate binding. The existence of both MST4 isoforms suggests that the MST4 kinase activity is highly regulated, and MST4a may function as a dominant-negative regulator of the MST4 kinase.