Sustained activation of the mitogen-activated protein (MAP) kinase cascade may be required for differentiation of PC12 cells. Comparison of the effects of nerve growth factor and epidermal growth factor

Identification and characterization of a new mammalian mitogen-activated protein kinase kinase, MKK2

Mitogen-activated protein (MAP) kinases are serine/threonine protein kinases activated by dual phosphorylation on threonine and tyrosine residues. A MAP kinase kinase (MKK1 or MEK1) has been identified as a dual-specificity protein kinase that is sufficient to phosphorylate MAP kinases p42mapk and p44mapk on the regulatory threonine and tyrosine residues. Because of the multiplicity of MAP kinase isoforms and the diverse circumstances and agonists leading to their activation, we thought it unlikely that a single MKK could accommodate this complexity. Indeed, two protein bands with MKK activity have previously been identified after renaturation following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We now report the molecular cloning and characterization of a second rat MAP kinase kinase cDNA, MKK2. MKK2 cDNA contains an open reading frame encoding a protein of 400 amino acids, 7 residues longer than MKK1 (MEK1). The amino acid sequence of MKK2 is 81% identical to that of MKK1, but nucleotide sequence differences occur throughout the aligned MKK2 and MKK1 cDNAs, indicating that MKK2 is the product of a distinct gene. MKK1 and MKK2 mRNAs are expressed differently in rat tissues. Both cDNAs when expressed in COS cells displayed the ability to phosphorylate and activate p42mapk and p44mapk, both MKK1 and MKK2 were activated in vivo in response to serum, and both could be phosphorylated and activated by the v-Raf protein in vitro. However, differences between MKK1 and MKK2 in sites of phosphorylation by proline-directed protein kinases predict differences in feedback regulation.

Complementation of byr1 in fission yeast by mammalian MAP kinase kinase requires coexpression of Raf kinase

Intracellular signalling from receptor tyrosine kinases in mammalian cells involves the activation of a signal cascade which includes p21ras and the protein kinases p74raf-1, MAP kinase kinase and MAP kinases. In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae the response to mating pheromones requires the Spk1 and KSS1/FUS3 kinases, which have sequence homology to vertebrate MAP kinases. The recent cloning of complementary DNAs for mammalian and frog MAP kinase kinases has shown that they are homologous to the S. pombe Byr1 (ref. 17) and S. cerevisiae STE7 (ref. 18) kinases, which have been proposed to function upstream of Spk1 and KSS1/FUS3, respectively. We have investigated whether these apparently similar kinase pathways are functionally conserved between vertebrates and S. pombe. We report here that expression of mammalian MAP kinase kinase alone fails to complement a byr1 mutant of S. pombe. When coexpressed with Raf kinase, however, MAP kinase kinase is activated by phosphorylation and the mating defect of the byr1 mutant is rescued. This suggests that the pathways are functionally homologous and that Raf kinase may directly phosphorylate and activate MAP kinase kinase.

Identification of a MAP kinase kinase kinase in phaeochromocytoma (PC12) cells

A MAP kinase kinase kinase (MAPKKK) was identified in phaeochromocytoma (PC12) cells which reactivated homogeneous MAP kinase kinase (MAPKK) from rabbit skeletal muscle that had been inactivated by incubation with protein phosphatase 2A. Reactivation was accompanied by stoichiometric phosphorylation of MAPKK on a serine residue(s). Following stimulation of PC12 cells with nerve growth factor and chromatography of the extracts on Mono Q, MAP kinase and MAPKK were detected as active phosphorylated enzymes, whereas MAPKKK was inactive and only activated after prolonged storage at 4 degrees C. The results suggest that the activation of MAPKKK by growth factors is likely to occur by a non-covalent mechanism.